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Phoenix666 writes:

Phys.org:

"We are coming to a point in our history in which we need to start looking for more space," Han Admiraal, a civil engineer with over two decades of experience in underground space, told AFP on the sidelines of this year's World Tunnel Congress.
...
"Underground spaces could easily be used for growing crops," he said, as he toured the cavernous Bourbon Tunnel, dug deep under the Italian city of Naples as a potential escape route for King Ferdinand II of Bourbon after the 1848 riots.

Scientific developments in areas like aquaponics—where vegetables and fish are farmed together—could help relieve the pressure on the food supply chain, and dramatically cut transport costs if such new farms were situated under cities.

Isn't excavation expensive?

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