Modern explorers are nothing like Ferdinand Magellan, whose maps were riddled with blank spaces. Today’s maps display the work of centuries of cartographers and thousands of satellites. Still, some areas remain unknown. Unnamed glaciers streak across the Alaskan wilderness, untamed peaks jut out from the Patagonian mountains, and unseen shores hug the Greenlandic north. Nearly every ocean has unswum waters with seafloors known only to the bottom-dwelling creatures that live there.