A lack of light
What life lurks in the deepest, darkest parts of our planet's oceans? These unexplored remote areas hold secrets about animal behavior that mankind has never seen. And because there are more questions than answers about life at the bottom of the ocean, our imaginations run wild with tales of sea serpents like the Kraken or the Loch Ness Monster.
But there are very real monster-like creatures living thousands of feet below the surface (like the bigfin reef squid pictured here) and they've adapted to their hostile environment over millions of years by taking on some unbelievably cool — and in some cases, scary — physical characteristics. Here are 15 rarely seen denizens of the deep.
Editor's note: This slideshow has been updated since it was originally published in August 2016.