First did not understand, a big black blob....then the blob dove to the bottom and I saw a big terrapin (I think) turtle. I let it enjoy my pool for a few hours, then photographed it, then I took the pool brush and politely but firmly asked him out. Boy, those critters are swift! And agile!
AMOF it was pretty fast on the dry land too. It went to my neighbour's.
I think it is 'cause we have a pond about 100 feet from my fence, and the fence is above ground a little bit.
I hope I do not need to worry much about what it can bring with it, but I asked my robot to sweep the floor and will shock it tonight just in case.