Filled with
WHAT!?
Now i am scared of it.
that is why you need to check it first. check it with your hand or something else that can tell you how safe it is.
if there are NO wildlife near it and the plantlife look dead. keep the hell away from it.
also the germs? Brain-eating amoeba, bacteria that causes Legionnaires' disease and many other kind of bacterias.
and even if it is SAFE. don't stay in there too long and make sure no animals will find an easy prey in you. "Mmm.. halfboiled human."
but don't worry. the ones in Iceland and Japan are rather safe and they do get checked often.