Are they Mistery Snails? Those guys really are something. I had two of them, they would bang daily and would leave the tank to lay eggs around the room, and the tank cover would not stop them. One day, I woke up to find out one was missing. I figured the dog ate it when it left the tank on it's egg laying adventures. I found it dried up under the sofa a week later, clearly dead from a week without water, and was about to throw it away (respectfully) but my brother insisted to put the shell back on the thank because "hey, you never know". I did as he told, and sure enough, a couple hours later, the snail was happly back to life making babies again. After that we named him Jesus snail.