Henry Bushnell for Yahoo:
They approached NFL stadium operators, and essentially asked: “Could we pull it off?”
...
They built more than an acre of pool deck some 10 feet above the stadium’s floor. The elevated “false deck,” made of “rubberized woven flooring,” allowed them to fit a 2.5-meters-deep pool without drilling into the stadium’s concrete base.
...
Working with the local fire department and an energy company, engineers tapped into a nearby hydrant, and pumped in some 1,200 gallons per minute. The water flowed through specially built pipes, from the concourse down over seats, to the stadium floor — which is two stories below street level.
After passing through regenerative media filters and UV-light chambers, which help with sanitation, it filled the pools. And after an Olympic team has been named, it will be pumped back out to the street.