This thing showed up one spring seemingly out of nowhere. In the fall I'd collected a few dozen pounds of beach sand from a local lake and dumped it in a 50 gallon aquarium. It wasn't used over the winter and in the spring I was getting it ready for use and filled it up with freezing cold tapwater and let it sit for a couple of days to settle out. It was fairly clean but there was this odd big leaf in there, dark brown, which was weird because I'd never seen a leaf in there before. I was just about to pull it out with my hands, wondering how the leaf got in there, it was just sand and only sand in the fall and there were no trees in that room, when suddenly the leaf swam away. I grabbed my camera and was able to take thse picture despite the fact I was shaking visibly.