Everyone who has a cistern looks forward to that day when they have to empty it and take the plunge into the depths of the earth to clean the gunk out. Even if you have a roof wash system, sediment inevitably builds up over time when your water comes from the sky.
I’m doing the dirty job of cleaning the cistern at Morel, one of the tiny houses at Dancing Rabbit Ecovillage. I’m pretty sure this cistern has never been cleaned, so it’s got some serious paleolithic buildup in the bottom.