Here's something I never knew.
I was watching a show about sharks with my son recently and a strange little shark was mentioned, the Cookiecutter shark. A small species of deep ocean dogfish the Cookiecutter shark has a bioluminescent underbelly and short, powerful jaws.
This shark also has a reputation for it's distinctive bite.
Bite damage (Source: NOAA)
It is known for biting out small plugs of flesh from large fish and whales as well as damaging undersea cables. But the thing I found interesting is that it has been known to attack nuclear submarine sonar domes. Here's a conversation from an Australian website on the topic.
I have to say the sea is full of some strange creatures, including the ones outside the pressure hull.