Glaciologists Andrea Fischer and Martin Stocker-Waldhuber, from the Austrian Academy of Sciences, inspect the ceiling of a natural glacier cavity of the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

Glaciologist Andrea Fischer, from the Austrian Academy of Sciences, stands at the entrance of a natural glacier cavity of the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

Glaciologist Andrea Fischer, from the Austrian Academy of Sciences, hikes towards the entrance of a natural glacier cavity of the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

Glaciologists Andrea Fischer and Martin Stocker-Waldhuber, from the Austrian Academy of Sciences, inspect the entrance of a natural glacier cavity of the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

Glaciologist Andrea Fischer, from the Austrian Academy of Sciences, enters a natural glacier cavity of the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

Glaciologists Andrea Fischer and Martin Stocker-Waldhuber, from the Austrian Academy of Sciences, explore a natural glacier cavity of the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

Glaciologist Andrea Fischer, from the Austrian Academy of Sciences, explores a natural glacier cavity of the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

Glaciologists Andrea Fischer and Martin Stocker-Waldhuber, from the Austrian Academy of Sciences, hike towards the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

Glaciologists Andrea Fischer and Martin Stocker-Waldhuber, from the Austrian Academy of Sciences, inspect a crack in the ceiling of a natural glacier cavity of the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

Glaciologists Andrea Fischer and Martin Stocker-Waldhuber, from the Austrian Academy of Sciences, inspect the ceiling of a natural glacier cavity of the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

Glaciologist Andrea Fischer, from the Austrian Academy of Sciences, explores a natural glacier cavity of the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

A general view of the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

Glaciologist Martin Stocker-Waldhuber, from the Austrian Academy of Sciences, explores a natural glacier cavity of the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

Glaciologists Andrea Fischer and Martin Stocker-Waldhuber, from the Austrian Academy of Sciences, explore a natural glacier cavity of the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

Glaciologists Andrea Fischer and Martin Stocker-Waldhuber, from the Austrian Academy of Sciences, inspect the entrance of a natural glacier cavity of Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

Glaciologists Andrea Fischer and Martin Stocker-Waldhuber, from the Austrian Academy of Sciences, explore a natural glacier cavity of the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

Glaciologist Martin Stocker-Waldhuber, from the Austrian Academy of Sciences, explores a natural glacier cavity of the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

Glaciologist Andrea Fischer, from the Austrian Academy of Sciences, explores a natural glacier cavity of the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner TPX IMAGES OF THE DAY

Glaciologist Andrea Fischer, from the Austrian Academy of Sciences, inspects a piece of ice inside a natural glacier cavity of the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

Glaciologists Andrea Fischer and Martin Stocker-Waldhuber, from the Austrian Academy of Sciences, inspect the entrance of a natural glacier cavity of the Jamtalferner glacier near Galtuer, Austria, October 15, 2021. Giant ice caves have appeared in glaciers accelerating the melting process faster than expected as warmer air rushes through the ice mass until it collapses. Picture taken October 15, 2021. REUTERS/Lisi Niesner

Historian Hanna Tychka explores a cave at the city sewage system where dozens of Jews were hiding from the Nazis during World War Two in Lviv, Ukraine September 25, 2021. Picture taken September 25, 2021. REUTERS/Gleb Garanich

Digger Oleksandr Ivanov and historian Hanna Tychka explore a cave at the city sewage system where dozens of Jews were hiding from the Nazis during World War Two in Lviv, Ukraine September 25, 2021. Picture taken September 25, 2021. REUTERS/Gleb Garanich

The entrance to a cave at Chhngauk Hill is pictured in Thala Borivat District, Steung Treng Province, Cambodia, August 31, 2021. Picture taken August 31, 2021. REUTERS/Cindy Liu

Researchers from the Institut Pasteur du Cambodge collect bat feces from a cave at Chhangauk Hill in Thala Borivat District, Steung Treng Province, Cambodia, August 31, 2021. Picture taken August 31, 2021. REUTERS/Cindy Liu

Red ocher markings which were painted on stalagmites by Neanderthals about 65,000 years ago, according to an international study, are seen in a prehistoric cave in Ardales, southern Spain, August 7, 2021. Picture taken August 7, 2021. REUTERS/Jon Nazca

A guide looks at red ocher markings which were painted on stalagmites by Neanderthals about 65,000 years ago, according to an international study, in a prehistoric cave in Ardales, southern Spain, August 7, 2021. Picture taken August 7, 2021. REUTERS/Jon Nazca

A negative hand stencil in black, airbrushed on a rock 36,000 years ago, is seen in the "Room of the Stars", in a prehistoric cave where red ocher markings were painted on stalagmites by Neanderthals about 65,000 years ago, according to an international study, in Ardales, southern Spain, August 7, 2021. Picture taken August 7, 2021. REUTERS/Jon Nazca

A negative hand stencil in black, airbrushed on a rock 36,000 years ago, is seen in the "Room of the Stars", in a prehistoric cave where red ocher markings were painted on stalagmites by Neanderthals about 65,000 years ago, according to an international study, in Ardales, southern Spain, August 7, 2021. Picture taken August 7, 2021. REUTERS/Jon Nazca

Visitors walk in the "Room of the Stars", in a prehistoric cave where red ocher markings were painted on stalagmites by Neanderthals about 65,000 years ago, according to an international study, in Ardales, southern Spain, August 7, 2021. Picture taken August 7, 2021. REUTERS/Jon Nazca

Red ocher markings which were painted on stalagmites by Neanderthals about 65,000 years ago, according to an international study, are seen in a prehistoric cave in Ardales, southern Spain, August 7, 2021. Picture taken August 7, 2021. REUTERS/Jon Nazca

A guide shows red ocher markings which were painted on stalagmites by Neanderthals about 65,000 years ago, according to an international study, in a prehistoric cave in Ardales, southern Spain, August 7, 2021. Picture taken August 7, 2021. REUTERS/Jon Nazca

The exterior of a prehistoric cave where red ocher markings were painted on stalagmites by Neanderthals about 65,000 years ago, according to an international study, is seen in Ardales, southern Spain, August 7, 2021. Picture taken August 7, 2021. REUTERS/Jon Nazca

A guide iluminates red ocher markings which were painted on stalagmites by Neanderthals about 65,000 years ago, according to an international study, in a prehistoric cave in Ardales, southern Spain, August 7, 2021. Picture taken August 7, 2021. REUTERS/Jon Nazca

Visitors walk in the "Room of the Stars", in a prehistoric cave where red ocher markings were painted on stalagmites by Neanderthals about 65,000 years ago, according to an international study, in Ardales, southern Spain, August 7, 2021. Picture taken August 7, 2021. REUTERS/Jon Nazca

Red ocher markings which were painted on stalagmites by Neanderthals about 65,000 years ago, according to an international study, are seen in a prehistoric cave in Ardales, southern Spain, August 7, 2021. Picture taken August 7, 2021. REUTERS/Jon Nazca

Red ocher markings which were painted on stalagmites by Neanderthals about 65,000 years ago, according to an international study, are seen in a prehistoric cave in Ardales, southern Spain, August 7, 2021. Picture taken August 7, 2021. REUTERS/Jon Nazca

Director of the prehistoric cave of Ardales, Pedro Cantalejo, 64, and Gerardo, a guide, stand in the "Room of the Stars", in a prehistoric cave where red ocher markings were painted on stalagmites by Neanderthals about 65,000 years ago, according to an international study, in Ardales, southern Spain, August 7, 2021. Picture taken August 7, 2021. REUTERS/Jon Nazca

Red ocher markings which were painted on stalagmites by Neanderthals about 65,000 years ago, according to an international study, are seen in a prehistoric cave in Ardales, southern Spain, August 7, 2021. Picture taken August 7, 2021. REUTERS/Jon Nazca

A view of a "cave" dwelling where Chinese President Xi Jinping used to stay, during a government-organised tour marking the 100th founding anniversary of the Communist Party of China, in Liangjiahe village, Shaanxi province, China May 31, 2021. Picture taken May 31, 2021. REUTERS/Ryan Woo

People visit a "cave" dwelling where Chinese President Xi Jinping used to stay, during a government-organised tour marking the 100th founding anniversary of the Communist Party of China, in Liangjiahe village, Shaanxi province, China May 31, 2021. Picture taken May 31, 2021. REUTERS/Ryan Woo

A wax figure of former Chinese chairman Mao Zedong is seen inside a "cave" dwelling at a tourism site during a government-organised tour marking the 100th founding anniversary of the Communist Party of China, in Yanan, Shaanxi province, China May 29, 2021. Picture taken May 29, 2021. REUTERS/Ryan Woo

An archeologist using a torch inspects the world's oldest limestone cave painting, thought to be dated to at least 45,500 years ago, which is decaying at a rapid pace due to climate change, in Maros regency, South Sulawesi province, Indonesia, May 27, 2021. Picture taken May 27, 2021. REUTERS/Abdul Rahman Muchtar

Archeologists gather while inspecting the world's oldest limestone cave painting, thought to be dated to at least 45,500 years ago, which is decaying at a rapid pace due to climate change, in Maros regency, South Sulawesi province, Indonesia, May 27, 2021. Picture taken May 27, 2021. REUTERS/Abdul Rahman Muchtar

Emmanuel Ndiema, head of archaeology at the National Museums of Kenya, talks to Reuters at a vault containing remains of a child buried around 78,000 years ago in Panga ya Saidi cave near Kenya's coast, at the National Museum of Kenya, in Nairobi, Kenya, May 12, 2021. Picture taken May 12, 2021. REUTERS/Baz Ratner

An employee displays remains of a child buried around 78,000 years ago in Panga ya Saidi cave near Kenya's coast, at the National Museum of Kenya, in Nairobi, Kenya, May 12, 2021. Picture taken May 12, 2021. REUTERS/Baz Ratner

Remains of a child buried around 78,000 years ago in Panga ya Saidi cave near Kenya's coast are shown at the National Museum of Kenya, in Nairobi, Kenya, May 12, 2021. Picture taken May 12, 2021. REUTERS/Baz Ratner

Emmanuel Ndiema, head of Archaeology at the National museums of Kenya holds boxes containing remains of a child buried around 78,000 years ago in Panga ya Saidi cave near Kenya's coast, at a vault in the National Museum of Kenya, in Nairobi, Kenya, May 12, 2021. Picture taken May 12, 2021. REUTERS/Baz Ratner

An employee holds a box containing remains of a child buried around 78,000 years ago in Panga ya Saidi cave near Kenya's coast, at a vault in the National Museum of Kenya, in Nairobi, Kenya, May 12, 2021. Picture taken May 12, 2021. REUTERS/Baz Ratner

Remains of a child buried around 78,000 years ago in Panga ya Saidi cave near Kenya's coast are seen at the National Museum of Kenya, in Nairobi, Kenya, May 12, 2021. Picture taken May 12, 2021. REUTERS/Baz Ratner

Roses are placed next to a grave on Mother's Day at Cave Hill Cemetery in Louisville, Kentucky, U.S., May 9, 2021. REUTERS/Amira Karaoud

People stand next to a grave on Mother's Day at Cave Hill Cemetery in Louisville, Kentucky, U.S., May 9, 2021. REUTERS/Amira Karaoud

A funeral wreath is seen next to a grave on Mother's Day at Cave Hill Cemetery, in Louisville, Kentucky, U.S. May 9, 2021. REUTERS/Amira Karaoud

A man stands next to a grave on Mother's Day at Cave Hill Cemetery, in Louisville, Kentucky, U.S. May 9, 2021. REUTERS/Amira Karaoud

Roses bouquets are placed next to graves on Mother's Day at Cave Hill Cemetery, in Louisville, Kentucky, U.S. May 9, 2021. REUTERS/Amira Karaoud

Mother's Day teddy bear is placed next to a grave at Cave Hill Cemetery Louisville, Kentucky, U.S. May 9, 2021. REUTERS/Amira Karaoud

A boat is seen through the small entrance of the Blue Grotto, one of the glamorous island of Capri's main tourist attractions that would have around 20-25 boats of people inside the cave during the summer season, but is instead empty and closed while the island awaits the return of tourists and the summer season despite the coronavirus disease (COVID-19), in Capri, Italy, April 28, 2021. Picture taken April 28, 2021. REUTERS/Yara Nardi

A general view of the Blue Grotto, one of the glamorous island of Capri's main tourist attractions that would have around 20-25 boats of people inside the cave during the summer season, but is instead empty and closed while the island awaits the return of tourists and the summer season despite the coronavirus disease (COVID-19), in Capri, Italy, April 28, 2021. Picture taken April 28, 2021. REUTERS/Yara Nardi

Vincenzo Iaccarino, 57, rows a boat inside the Blue Grotto, one of the glamorous island of Capri's main tourist attractions that would have around 20-25 boats of people inside the cave during the summer season, but is instead empty and closed while the island awaits the return of tourists and the summer season despite the coronavirus disease (COVID-19), in Capri, Italy, April 28, 2021. Picture taken April 28, 2021. REUTERS/Yara Nardi

Vincenzo Iaccarino, 57, rows a boat inside the Blue Grotto, one of the glamorous island of Capri's main tourist attractions that would have around 20-25 boats of people inside the cave during the summer season, but is instead empty and closed while the island awaits the return of tourists and the summer season despite the coronavirus disease (COVID-19), in Capri, Italy, April 28, 2021. Picture taken April 28, 2021. REUTERS/Yara Nardi

Vincenzo Iaccarino, 57, rows a boat inside the Blue Grotto, one of the glamorous island of Capri's main tourist attractions that would have around 20-25 boats of people inside the cave during the summer season, but is instead empty and closed while the island awaits the return of tourists and the summer season despite the coronavirus disease (COVID-19), in Capri, Italy, April 28, 2021. Picture taken April 28, 2021. REUTERS/Yara Nardi