A kosher mikveh is built on rainwater collected in the bor in a manner that does not invalidate it. In this article: which rainwater qualifies, what hashaka is, the role of the otzar, and why any change to the plumbing requires a posek's ruling.
All information on this site is reviewed by the organization's poskim. It is intended for general familiarity and does not replace asking a rav — for any practical question, please consult your rav.