DARPA tweeted on Wednesday that within the next 48 hours it must find a "human-made underground environment spanning several city blocks with complex layout & multiple stories, including atriums, tunnels & stairwells. Spaces that are currently closed off from pedestrians or can be temporarily used for testing are of interest."

I'm imagining something along the lines of the underground world used by the tethered in the movie Us, though it also sounds like abandoned underground malls, bunkers, cities, and as-to-yet undiscovered sites of lost civilizations may qualify, so long as the space is found by 5 PM Friday (Eastern Time; the government's hours are very rigid.)

DARPA's "Underground Experimentation Request for Information" sheds slightly more information: "DARPA is interested in understanding the state of the art in innovative technologies that may enable future solutions to rapidly map, navigate, and search unknown complex subterranean environments to locate objects of interest." Simply put, it sounds like DARPA wants to play a massive underground game of hide-and-seek because "the subterranean domain" is becoming "increasingly relevant for global security and disaster-related search and rescue missions."