What is the strangest place that an animal can live in?

My favorite for a strange habitat is for the pilot fish. A carnivorous fish that's one of a group of 'cleaner fish' that feeds on the scraps from larger carnivores by hanging out with them, that includes hanging out with sharks. Pilot fish follow them around and pick bits from their bodies and even swim around in the sharks mouth to pick their teeth clean while the shark holds its mouth open to let them do their job.