6. Box Jellyfish
Often found floating (or moving at speeds close to five miles per hour) in Indo-Pacific waters, these transparent, nearly invisible invertebrates are considered by the National Oceanic and Atmospheric Administration as the most venomous marine animal in the world. Their namesake cubic frames contain up to 15 tentacles at the corners, with each growing as much as 10 feet long, all lined with thousands of stinging cells—known as nematocysts—that contain toxins that simultaneously attack the heart, nervous system, and skin cells. While antivenins do exist, the venom is so potent and overwhelming that many human victims, of the hundreds of reported fatal encounters each year, have been known to go into shock and drown or die of heart failure before reaching shore. Even if you are lucky enough to make it to the hospital and receive the antidote, survivors can sometimes experience considerable pain for weeks afterward and bear nasty scars from the creature’s tentacles.