This here's concept art for BNKR Arquitectura's Earthscraper, an inverted skyscraper shoved deep under the surface of Mexico City. Click on the above image to see a massive cross-section.

Explains the firm of this 300-meter-deep structure:

The Historic Center of Mexico City is in a desperate need of a programmatic make-over. New infrastructure, office, retail and living space is required but no empty plots are available. Federal and local laws prohibit demolishing historic buildings and height regulations limit new structures to eight stories. The Earthscraper is the Skyscrapers antagonist in a historic urban landscape where the latter is condemned and the preservation of the built environment is the paramount ambition. It preserves the iconic presence of the city square and the existing hierarchy of the buildings that surround it. It is an inverted pyramid with a central void to allow all habitable spaces to enjoy natural lighting and ventilation.

To conserve the numerous activities that take place on the city square year round (concerts, political manifestations, open-air exhibitions, cultural gatherings, military parades.), the massive hole is covered with a glass floor that allows the life of the Earthscraper to blend with everything happening on top.

I'm not sure how useful the Earthscraper would be in Mexico City (what with earthquakes and the sinking), but it's definitely a fascinating idea on par with building an arcology in a Siberian diamond mine. Anyone want to commute 220 meters underground to their office?

[Via Geekologie. Thanks, a.seivewright!]