Aquifer

Welltower gets most of its water from three huge underground lakes near it. They are called aquifers, lakes, or reservoirs.

The most significant aquifer to the story is located on level 437. It's where Doctor Herstand keeps her hospital; Kaehl visits it several times. This aquifer is breached by an earthquake in book 2, causing a massive flood and effectively destroying large parts of Welltower. The water rose all the way to level 618.