Something about a double level swimming pool.
One level on top of the other & see through below & above.
Swim & watch or compete against someone or in a race with many swimmers.
Swap top level to lower level etc. For race(s) etc.
View the below swimmer in race(s).
Designers use your imagination please.
A gap for oxygen in the lower swimming pool or just a double level swimming pool with see through glass, made from silica, which is a natural material.