Sump

A sump can also be an infiltration basin used to manage surface runoff water and recharge underground aquifers.

The sump sits below the main tank and is used as a filter, as well as a holding place of unsightly equipment such as heaters and protein skimmers.

The main advantage of having a sump plumbed into an aquarium is the increase of water in the system, making it more stable and less prone to fluctuations of pH and salinity.

[3] In caving/potholing terminology, a sump is a permanently flooded section of a cave, such that the caver must submerge under water to reach the other side.

In medieval cosmology, the sump was the center of the cosmos, where the dregs and filth descended, with the celestial sphere far exalted above the world of fallen man.