According to National Oceanic and Atmospheric Administration, the box jellyfish is one of the most dangerous marine animals in the world. These threatening fish are found in the Indo-Pacific waters north of Australia. The scary part about the box jellyfish is that they are almost invisible! They also have up to 15 tentacles. Each tentacle grows up to 10 feet long and each tentacle is lined with stingers that contain toxins. These dangerous toxins attack the heart, nervous system and skin cells. Most victims drown after being stung.