Underground City in Diamond Mine, Eco-City Project 2020 by AB Elis Ltd

Eco-city 2020 is a proposal for the rehabilitation of the Mirniy industrial zone in Eastern Siberia, Russia designed by the innovative architectural studio AB Elis Ltd.
The city, named Eco-city 2020, would be constructed inside the Mir diamond mine, the second largest excavated hole on the planet. It's a quarter-mile wide at the top and over 1,700 feet deep, which is so big that air flowing into the hole can actually suck helicopters out of the sky. If the project gets going, the mine would be completely covered over with a glass dome to protect the city from the weather in Siberia (which is apparently lousy almost all the time), and solar cells embedded in the dome would provide power for the entire structure.

Eco-city would be constructed of multiple levels, with a huge central core. The main floor would hold parks and recreation areas, with residential areas terracing up around the walls of the mine. Underneath would be space for vertical farms and forests, subsiding on light piped down the central core. An estimated 100,000 people would be able to live in Eco-city, and architects are hoping that it would help to attract tourists to Eastern Siberia.

So, what’s the likelihood of Eco-city actually being constructed? Probably not great, at least not anytime soon. At the same time, settlement like this (on a much smaller scale) might be the perfect design for a colony in a crater on the Moon or Mars.