The surface area of an experimental mine at the School of Mines is seen in Idaho Springs, Colorado, U.S., December 9, 2021. REUTERS/Kevin Mohatt 

Indian Army's Surface Mine Clearing System (SMCS) is displayed during the Republic Day parade in New Delhi, India, January 26, 2019. REUTERS/Altaf Hussain 

Miners leave "shaft 7", some 1,200 metres below surface, following their shift at Germany's last active coal mine Prosper-Haniel of the RAG foundation in Bottrop, western Germany, September 17, 2018, that will be officially closed on December 21, 2018. Picture taken September 17, 2018. REUTERS/Wolfgang Rattay 

Miners leave "shaft 7", some 1,200 metres below surface, following their shift at Germany's last active coal mine Prosper-Haniel of the RAG foundation in Bottrop, western Germany, September 17, 2018, that will be officially closed on December 21, 2018. Picture taken September 17, 2018. REUTERS/Wolfgang Rattay 

Miners leave "shaft 7", some 1,200 metres below surface, following their shift at Germany's last active coal mine Prosper-Haniel of the RAG foundation in Bottrop, western Germany, September 17, 2018, that will be officially closed on December 21, 2018. Picture taken September 17, 2018. REUTERS/Wolfgang Rattay 

Miners leave "shaft 7", some 1,200 metres below surface, following their shift at Germany's last active coal mine Prosper-Haniel of the RAG foundation in Bottrop, western Germany, September 17, 2018, that will be officially closed on December 21, 2018. Picture taken September 17, 2018. REUTERS/Wolfgang Rattay 

A miner leaves "shaft 7", some 1,200 metres below surface, following his shift at Germany's last active coal mine Prosper-Haniel of the RAG foundation in Bottrop, western Germany, September 17, 2018, that will be officially closed on December 21, 2018. Picture taken September 17, 2018. REUTERS/Wolfgang Rattay 

Miners enter the mine cage at "shaft 6", some 1,000 metres below surface, in Germany's last active coal mine Prosper-Haniel of the RAG foundation in Bottrop, western Germany, December 11, 2018, that will be officially closed on December 21, 2018. Picture taken December 11, 2018. REUTERS/Wolfgang Rattay 

Miners wait in a mine cage to be lifted up from "shaft 6", some 1,000 metres below surface, in Germany's last active coal mine Prosper-Haniel of the RAG foundation in Bottrop, western Germany, December 11, 2018, that will be officially closed on December 21, 2018. Picture taken December 11, 2018. REUTERS/Wolfgang Rattay 

Miners leave the mine cage at "shaft 6", some 1,000 metres below surface, in Germany's last active coal mine Prosper-Haniel of the RAG foundation in Bottrop, western Germany, December 11, 2018, that will be officially closed on December 21, 2018. Picture taken December 11, 2018. REUTERS/Wolfgang Rattay 

A driver steers a personel train in "shaft 6", some 1,000 metres below surface, in Germany's last active coal mine Prosper-Haniel of the RAG foundation in Bottrop, western Germany, December 11, 2018, that will be officially closed on December 21, 2018. Picture taken December 11, 2018. REUTERS/Wolfgang Rattay 

An empty lorry is seen in "shaft 6", some 1,000 metres below surface, in Germany's last active coal mine Prosper-Haniel of the RAG foundation in Bottrop, western Germany, December 11, 2018, that will be officially closed on December 21, 2018. Picture taken December 11, 2018. REUTERS/Wolfgang Rattay 

The empty "shaft 6", some 1,000 metres below surface, is pictured in Germany's last active coal mine Prosper-Haniel of the RAG foundation in Bottrop, western Germany, December 11, 2018, that will be officially closed on December 21, 2018. Picture taken December 11, 2018. REUTERS/Wolfgang Rattay 

Marcel Pawlinka, a 24-year-old miner, poses in "shaft 6", some 1,000 metres below surface, in Germany's last active coal mine Prosper-Haniel of the RAG foundation in Bottrop, western Germany, December 11, 2018, that will be officially closed on December 21, 2018. Picture taken December 11, 2018. REUTERS/Wolfgang Rattay 

Marcel Pawlinka, a 24-year-old miner, poses in "shaft 6", some 1,000 metres below surface, in Germany's last active coal mine Prosper-Haniel of the RAG foundation in Bottrop, western Germany, December 11, 2018, that will be officially closed on December 21, 2018. Picture taken December 11, 2018. REUTERS/Wolfgang Rattay 

Marcel Pawlinka, a 24-year-old miner poses in "shaft 6" some 1,000 metres below surface in Germany's last active coal mine Prosper-Haniel of the RAG foundation in Bottrop, western Germany, December 11, 2018, that will be officially closed on December 21, 2018. Picture taken December 11, 2018. REUTERS/Wolfgang Rattay 

Two miners are pictured at "shaft 7", some 1,200 metres below surface, in Germany's last active coal mine Prosper-Haniel of the RAG foundation in Bottrop, western Germany, September 17, 2018, that will be officially closed on December 21, 2018. Picture taken September 17, 2018. REUTERS/Wolfgang Rattay 

A miner walks past Saint Barbara, the Holy Helpers of the miners, at "shaft 6", some 1,000 metres below surface, in Germany's last active coal mine Prosper-Haniel of the RAG foundation in Bottrop, western Germany, December 11, 2018, that will be officially closed on December 21, 2018. Picture taken December 11, 2018. REUTERS/Wolfgang Rattay 

Two miners walk through "shaft 6", some 1,000 metres below surface, in Germany's last active coal mine Prosper-Haniel of the RAG foundation in Bottrop, western Germany, December 11, 2018, that will be officially closed on December 21, 2018. Picture taken December 11, 2018. REUTERS/Wolfgang Rattay 

A miner rides on a so-called "rail bicycle" at "shaft 6", some 1,000 metres below surface in Germany's last active coal mine Prosper-Haniel of the RAG foundation in Bottrop, western Germany, December 11, 2018, that will be officially closed on December 21, 2018. Picture taken December 11, 2018. REUTERS/Wolfgang Rattay 

A pile of salt is seen in a salt pan at Xwejni, outside the village of Zebbug on the island of Gozo, Malta September 4, 2018. Picture taken September 4, 2018. REUTERS/Darrin Zammit Lupi 

Salt flakes are seen in a salt pan at Xwejni, outside the village of Zebbug on the island of Gozo, Malta September 4, 2018. Picture taken September 4, 2018. REUTERS/Darrin Zammit Lupi 

A pile of salt is seen in a salt pan at Xwejni, outside the village of Zebbug on the island of Gozo, Malta September 4, 2018. Picture taken September 4, 2018. REUTERS/Darrin Zammit Lupi 

Homes built on a former surface strip coal mine sit above piles of coal in Norton, Virginia, U.S., May 18, 2018. Picture taken May 18, 2018. REUTERS/Brian Snyder 

A man golfs on a course built on the site of a former strip surface coal mine in Jenkins, Kentucky, U.S., May 20, 2018. REUTERS/Brian Snyder SEARCH "BLACK LUNG" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. 

Retired coal miner James Marcum, who has complicated black lung disease, stands for a chest x-ray at the Stone Mountain Health Services in St. Charles, Virginia, U.S., May 18, 2018. Marcum worked at a surface strip mine for 20-25 years, but has not yet been approved for benefits for his black lung disease. REUTERS/Brian Snyder SEARCH "BLACK LUNG" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. TPX IMAGES OF THE DAY. 

Nurse Melissa Muse checks the blood pressure of retired coal miner James Marcum, who has complicated black lung disease, during an exam at the Stone Mountain Health Services in St. Charles, Virginia, U.S., May 18, 2018. Marcum worked at a surface strip mine for 20 to 25 years, but has not yet been approved for benefits for his black lung disease. REUTERS/Brian Snyder SEARCH "BLACK LUNG" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. 

Roger Featherstone of the Arizona Mine Reform Coalition points to the Resolution Copper mine in the Oak Flat area near Superior, Arizona, June 13, 2017. In a federal land exchange, Resolution Copper acquired control over 2,400 acres in and around Oak Flat and plans to extract ore from deep below the surface. Picture taken June 13, 2017. REUTERS/Nancy Wiechec TPX IMAGES OF THE DAY 

Roger Featherstone of the Arizona Mine Reform Coalition stands near a spring-fed waterhole in the Oak Flat recreation area near Superior, Arizona, June 13, 2017. In a federal land exchange, Resolution Copper acquired control over 2,400 acres in and around Oak Flat and plans to extract ore from deep below the surface. Picture taken June 13, 2017. REUTERS/Nancy Wiechec 

Stakes mark a mining claim in the Oak Flats area near Superior, Arizona, June 13, 2017. In a federal land exchange, Resolution Copper Mining acquired control over 2,400 acres in and around Oak Flat and plans to extract ore from deep below the surface. Picture taken June 13, 2017. REUTERS/Nancy Wiechec 

Roger Featherstone of the Arizona Mine Reform Coalition hikes to one of the organization's wildlife cameras in the Oak Flat area near Superior, Arizona, June 13, 2017. In a federal land exchange, Resolution Copper acquired control over 2,400 acres in and around Oak Flat and plans to extract ore from deep below the surface. Picture taken June 13, 2017. REUTERS/Nancy Wiechec 

The Apache Leap rock formation is seen looking east from Main Street in Superior, Arizona, June 13, 2017. In a federal land exchange, Resolution Copper Mining acquired control over 2,400 acres in the adjacent Oak Flat recreation area and plans to extract ore from deep below the surface. Picture taken June 13, 2017. REUTERS/Nancy Wiechec 

An Arizona hedgehog cactus, an endangered variety, is seen in the Oak Flat recreation area outside Superior, Arizona, June 13, 2017. In a federal land exchange, Resolution Copper Mining acquired control over 2,400 acres in and around Oak Flat and plans to extract ore from deep below the surface. Picture taken June 13, 2017. REUTERS/Nancy Wiechec 

Roger Featherstone of the Arizona Mine Reform Coalition checks one of the organization's wildlife cameras in the Oak Flat area near Superior, Arizona, June 13, 2017. In a federal land exchange, Resolution Copper Mining acquired control over 2,400 acres in and around Oak Flat and plans to extract ore from deep below the surface. Picture taken June 13, 2017. REUTERS/Nancy Wiechec 

The stalk of an agave plant displays yellow blooms in the Oak Flat area near Superior, Arizona, June 13, 2017. In a federal land exchange, Resolution Copper acquired control over 2,400 acres in and around Oak Flat and plan to extract ore from deep below the surface. Picture taken June 13, 2017. REUTERS/Nancy Wiechec 

Resolution Copper Mining's east plant is seen from the Oak Flat recreation area near Superior, Arizona, June 13, 2017. In a federal land exchange, Resolution Copper acquired control over 2,400 acres in and around Oak Flat and plans to extract ore from deep below the surface. Picture taken June 13, 2017. REUTERS/Nancy Wiechec 

Roger Featherstone of the Arizona Mine Reform Coalition walks from an overlook in the Oak Flat recreation area near Superior, Arizona, June 13, 2017. In a federal land exchange, Resolution Copper Mining acquired control over 2,400 acres in and around Oak Flat and plans to extract ore from deep below the surface. Picture taken June 13, 2017. REUTERS/Nancy Wiechec 

A cholla cactus displays magenta blooms in the Oak Flat recreation area near Superior, Arizona, June 13, 2017. In a federal land exchange, Resolution Copper Mining acquired control over 2,400 acres in and around Oak Flat and plans to extract ore from deep below the surface. Picture taken June 13, 2017. REUTERS/Nancy Wiechec 

A forest service kiosk gives information at the Oak Flat Campground near Superior, Arizona, June 13, 2017. In a federal land exchange, Resolution Copper Mining acquired control over 2,400 acres in and around Oak Flat and plans to extract ore from deep below the surface. Picture taken June 13, 2017. REUTERS/Nancy Wiechec 

Roger Featherstone of the Arizona Mine Reform Coalition looks out toward the Resolution Copper mine in the Oak Flat area near Superior, Arizona, June 13, 2017. In a federal land exchange, Resolution Copper acquired control over 2,400 acres in and around Oak Flat and plans to extract ore from deep below the surface. Picture taken June 13, 2017. REUTERS/Nancy Wiechec 

An agave plant displays yellow blooms in the Oak Flat area near Superior, Arizona, June 13, 2017. In a federal land exchange, Resolution Copper Mining acquired control over 2,400 acres in and around Oak Flat and plans to extract ore from deep below the surface. Picture taken June 13, 2017. REUTERS/Nancy Wiechec 

FILE PHOTO: A truck travels to a depth of 516 metres below the surface to collect ore at the Chibuluma copper mine in the Zambian copperbelt region, January 17, 2015. REUTERS/Rogan Ward/File Photo 

A coal miner worker shakes hands with U.S. President Donald Trump speaks as he prepares to sign Resolution 38, which nullfies the Òstream protection ruleÓ, The rule addresses the impacts of surface coal mining operations on surface water, groundwater, and the productivity of mining operations sites, at the White House in Washington, U.S., February 16, 2017. REUTERS/Carlos Barria 

U.S. President Donald Trump signs H.J. Resolution 38, which nullfies the Òstream protection ruleÓ, The rule addresses the impacts of surface coal mining operations on surface water, groundwater, and the productivity of mining operations sites, at the White House in Washington, U.S., February 16, 2017. REUTERS/Carlos Barria 

FILE PHOTO -- The John Amos coal-fired power plant is seen behind a home in Poca, West Virginia May 18, 2014. With coal production slowing due to stricter environmental controls, the availability of natural gas and a shift to surface mining, the state's coal country has been hit hard with job losses and business closures. Picture taken May 18, 2014. REUTERS/Robert Galbraith/File Photo 

FILE PHOTO -- A dragline excavator moves surface dirt into a 240 ton haul truck during a tour of Peabody Energy's North Antelope Rochelle coal mine near Gillette, Wyoming, U.S. June 1, 2016. The company filed for Chapter 11 bankruptcy in April. REUTERS/Kristina Barker/File Photo 

ATTENTION EDITORS - VISUALS COVERAGE OF SCENES OF DEATH OR INJURY Relatives and friends bring to the surface the body of a miner after recovering it from Johannesburg's oldest gold mine in Langlaagte, South Africa, September 13, 2016. REUTERS/Siphiwe Sibeko 

Suspected illegal miners walk after they surfaced from an underground Johannesburg's oldest gold mine in Langlaagte, South Africa September 12, 2016. REUTERS/Siphiwe Sibeko 

Suspected illegal miners look on after they surfaced from an underground Johannesburg's oldest gold mine in Langlaagte, South Africa September 12, 2016. REUTERS/Siphiwe Sibeko 

A suspected illegal miner (C) is welcomed by friends and family members as he surfaces from an underground Johannesburg's oldest gold mine in Langlaagte, South Africa September 12, 2016. REUTERS/Siphiwe Sibeko 

A police officer gestures to a suspected illegal miner after he surfaced from an underground Johannesburg's oldest gold mine in Langlaagte, South Africa September 12, 2016. REUTERS/Siphiwe Sibeko 

A dragline excavator moves surface dirt into a 240 ton haul truck during a tour of Peabody Energy's North Antelope Rochelle coal mine near Gillette, Wyoming, U.S. June 1, 2016. The company filed for Chapter 11 bankruptcy in April. REUTERS/Kristina Barker 

METALS-LMEWEEK/ELNINO - Map with overlays copper, nickel and bauxite mines on August 2015 surface temperature anomaly. (SIN05) 

A guide and a visitor stand in front of a rock face with dinosaur track ways at the Cal Orcko paleontologic site at the FANCESA limestone quarry in Sucre, Bolivia, April 29, 2015. Frequent landslides at the paleontologic site of Cal Orcko constantly reveal new track ways, with some belonging to new species, according to local media. REUTERS/David Mercado TPX IMAGES OF THE DAY

A truck exits the mine after collecting ore from 516 metres below the surface at the Chibuluma copper mine in the Zambian copperbelt region, January 17, 2015. REUTERS/Rogan Ward (ZAMBIA - Tags: BUSINESS COMMODITIES) 

A truck exits the mine after collecting ore from 516 metres below the surface at the Chibuluma copper mine in the Zambian copperbelt region, January 17, 2015. REUTERS/Rogan Ward (ZAMBIA - Tags: BUSINESS COMMODITIES) 

A truck exits the mine after collecting ore from 516 metres below the surface at the Chibuluma copper mine in the Zambian copperbelt region, January 17, 2015. REUTERS/Rogan Ward (ZAMBIA - Tags: BUSINESS COMMODITIES) 

A truck travels to a depth of 516 metres below the surface to collect ore at the Chibuluma copper mine in the Zambian copperbelt region, January 17, 2015. REUTERS/Rogan Ward (ZAMBIA - Tags: BUSINESS COMMODITIES)