This unsettling creature is called Eunice aphroditois, or colloquially the Bobbit worm. These critters can grow up to three meters long and have pincers capable of slicing its (sometimes larger) prey right in half. Also? It injects a toxin into its prey to make it easier to digest. Yum.
The worm keeps itself buried in the sand or gravel at the bottom of the sea, only allowing its five tiny antennae to stick up out of the silt. If something swims or crawls along that disturbs one of the antennae, the worm springs up out of the ground and grabs whatever passerby happened to be unlucky that day. They're found throughout the warmer parts of the Indian, Pacific, and Atlantic Oceans.
Who needs Shark Week to be terrified? Just show us some aquatic predatory worms. This is the stuff of nightmares.
(h/t Yonatan Zunger)