Well, it's Shark Week, and apparently the sharks aren't happy. According to a Google engineer, the company has to wrap its undersea fiber optics cable in kevlar to protect them against shark attacks.
See for yourself:
It's not that sharks are pissed about their portrayal in the media and want to fight back. Theories as to why they do this range from sharks being attracted by the electro-magnetic fields that are leaking from the cables, which they might be able to detect, or just good old curiosity, making them nibble on random things that they find.
Either way, unprotected cables can easily be damaged by shark teeth, and doing repairs at the bottom of the sea isn't exactly easy...
While, as previously pointed out, humans are millions of times more dangerous for sharks than sharks are for humans, they still try to eat human things from time to time, and not just undersea cables. Here's a great white trying to eat a remote-controlled robot: