Sounds like a fresh water cistern used to catch roof runoff for domestic needs.
Report what you saw, any deficiencies, or possible dangers.
I found one by accident while crawling along on my belly in the dark crawl space, but it was much deeper an larger and the top was low enough to have fallen in, kind of a "WHOA!" moment.

__________________
Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002