This blog was written by Roger Di Silvestro, a field correspondent for Ocean Conservancy.

As Valentine’s Day nears, let’s interrupt our thoughts about love, roses, and chocolates and turn to a closely related subject: whales.

Yes, whales.

Before there is a Valentine’s Day, there is World Whale Day—a celebration of some of the planet’s most-fascinating, well-loved, and yet elusive creatures, looming large in the popular imagination but still in many ways a scientific mystery. Established on Maui in 1980 to remind people about whales, their lives, and their plight, World Whale Day has been celebrated ever since with parades and various whale-focused events.

In recognition of World Whale Day, here are 6 facts about some of the largest animals that ever lived.

1. Let’s start with the most-basic point—whales are mammals. In his classic work Moby Dick, or The Whale, Herman Melville concluded that whales are fish, proving you can write a Great American Novel and still get your facts wrong. With flippers, fins, and a torpedo-shaped body, whales look like fish, but forget about it: they evolved from an ancient, even-toed hoofed animal, making them relatives of deer, sheep, and gazelles, not guppies, tuna, or whale sharks, which, as the name suggests, are sharks. About 50 million years ago a now-extinct, semi-aquatic mammal species split off in two evolutionary directions, giving rise to the hippopotamus on the one hand and whales on the other, making the hippo the closest living relative of whales. Whales share the signature traits of all mammals: they breathe air, produce milk for their young, and grow hair, however scantly.

2. Scientists divide whales into two general groups, toothed and baleen. The latter lack teeth. Instead, their mouths are fringed with plates made of keratin, same as our fingernails. These plates are the baleen that gives the group its name. Baleen whales are filter feeders, taking in massive quantities of water teeming with such sea life as shrimp, krill, and small fish, forcing out the water through the baleen, and swallowing whatever is held back (filtered out).

3. The blue whale is the largest animal that ever lived, weighing up to 200 tons, more than twice the estimated weight of the largest known dinosaur. However, at no more than 110 feet long, the blue whale is at least 20 feet shorter than the longest dinosaur known, Argentinosaurus huinculensis (also the heaviest, at up to 99 tons). In any event, an adult blue whale is built on a massive scale.

4. The smallest whale species is the dwarf sperm whale, generally less than 8.5 feet long and weighing as little as 300 pounds. Other small whales include the white beluga of the Arctic and the St. Lawrence River and the narwhal, another species of cold, northern seas. Both grow to about 18 feet and 3,500 pounds, small enough to fall prey to polar bears lying in wait on sea ice near whale breathing holes. The narwhal is famed as a tusked whale, males growing a single, spiral tusk that can reach 11 feet long (males rarely grow two; about one in six females grows a tusk). The narwhal tusk gave rise to the legend of the unicorn and was traded for medicinal and magical purposes in medieval times.

5. Sperm whales use sonar (echolocation) to hunt prey and to sense the world around them. The sound waves they emit are so powerful that human divers swimming near the whales can feel the pulses. The sound bounces off objects in the water and returns to the whale, whose brain creates an image based on the signals. (The sperm whale is well equipped for sonar interpretation—it has the largest brain on earth, weighing about 17 pounds in an adult). Sonar images are precise enough that sperm whales can survive even if blind.

7. Whales are among the world’s deepest divers. When hunting squid, a sperm whale may spend as much as an hour on a dive to more than 3,000 feet, where the temperature hovers at 36 degrees F and the pressure is more than 1,400 pounds per square inch. Impressive, but not if you happen to be a Cuvier’s beaked whale—scientists recently observed one diving to about 10,000 feet (nearly two miles) and staying under for 138 minutes, a record for both length and depth.