A pool party for the hydromies

I was curious how the effect was created and found this: https://www.ignant.com/2016/01/07/an-illusory-swimming-pool-by-leandro-erlich/

Under a 10cm layer of water, the rest of the "pool" is a walk-in art exhibit. There are lots of photos in that link from both above and below the water. It's a permanent exhibit in a Japanese museum of contemporary art.