Solutions to the biggest threats facing our planet lie underground, according to experts who insist that climate change, overpopulation and food shortages can all be tackled by going subterranean.
"We are coming to a point in our history in which we need to start looking for more space," Mr Han Admiraal, a civil engineer with more than two decades of experience in underground space, told AFP on the sidelines of this year's World Tunnel Congress.

Efforts to meet seven of the United Nations' 17 sustainable development goals - from cleaning up pollution-clogged metropolises to ending world hunger - could be given a big boost by repurposing spaces below street level, he said.