Filling in an old septic tank /cistern
if your NOT filling with gravel etc,,,make DARN sure kids cant crawl under deck and fall in,,,they are TOO valuable to lose this way!! Most of those walls are real suspect and OLD, and may break in unless filled. I doubt anyone could survive long in cold ground water,,,let alone a kid. I could see gravel filled and drain out into a better suited tank for garden water.