The ocean covers about 70 percent of our planet, and it is still full of surprises. Here are seven facts about sea life that sound like science fiction but are completely true.
1. Octopuses Have Three Hearts and Blue Blood: Two hearts pump blood through the gills, and the third sends it to the rest of the body. Their blood is blue because it uses a copper-based molecule called hemocyanin to carry oxygen.
2. Sea Otters Hold Hands While They Sleep: Rafts of sea otters often link paws or wrap themselves in kelp so they don't drift apart while napping on the water.
3. There's a Jellyfish That Can "Reverse" Its Age: Turritopsis dohrnii, nicknamed the immortal jellyfish, can transform back into its juvenile polyp stage when stressed or injured, essentially starting its life cycle over.
4. A Blue Whale's Heart Is the Size of a Bumper Car: The largest animal ever known has a heart that can weigh around 400 pounds, and its heartbeat can be detected from miles away.
5. Seahorse Dads Give Birth: Female seahorses deposit their eggs into a pouch on the male, who carries them until hundreds of tiny seahorses pop out.
6. Most of the Seafloor Has Never Been Seen Up Close: We have more detailed maps of the surface of Mars than of much of the ocean floor here on Earth.
7. Starfish Don't Have Brains (or Blood): Sea stars move water through their bodies instead of blood, and they can regrow lost arms.
Next time you're at the beach, remember: there's a whole alien world just past the waves!