The T'vilah ("immersion") service takes place within the mikveh (literally, "a collection of water"), which has been part of Jewish life since the time of Moses.
G-d commanded that the people of Israel should separate themselves from uncleannes by what they ate, by how they acted, and by the recognition of what was impure and what was pure.
The mikveh was designed to be a place of ritual rather than of physical cleansing. The pool itself was usually 5 to 6 feet in length, 3 to 4 feet in width, and 5 to 6 feet in depth. It was cut into the ground and was entered by means of descending stairs. Water was brought to it, and it was used only for this purpose.