The second biggest swimming pool in the universe, the biggest being an entire planet so that's a bit unfair.
People from all over the galaxy have been visiting this pool for centuries now. People have put their towels on seats to occupy them for several years in a row, and minor wars have been fought over the use of the whirlpool. There are some talks of entire generations basicly living in Humidum and developping some kind of gills over the years, so most of the regulars are now water breathers.
Hovering over an ocean planet, there is never a shortage of the water supply. But when the chemicals to keep the pool run out we run into all kinds of problems. Let's see how a swimming pool deals with half a million kids doing their number ones and twos without chemicals to keep it clean.
Weekend project, took 7 hours from start to finish