All sorts of monsters live in this perpetual darkness: colossal squid lie in ambush, and hungry sperm whales prowl. Even if you're as big and powerful as a long-finned pilot whale, there's always the possibility of running into kraken.

So perhaps we shouldn't be surprised that pilot whales like to stick close together when they plunge into the murky depths. A new study shows that they stay within a few metres of each other as they dive, and even stroke each other with their flippers. After all, if you're going to venture into the darkness, it helps to have a hand to hold.

Nested families

Pilot whales – including the long-finned sort – aren't really whales. Like their relatives, the killer whales, they're actually dolphins.