The Sea Monster is a giant purple octopus, with massive tentacles, razor-sharp teeth and two red glowing eyes.
The Sea Monster was accidentally freed from the zoo of Atlantis when Tom leaned on the glass wall in front of its cage. The glass shattered and the creature was released. Its first victim was Tom, whom it angrily sideswiped with a tentacle, making his life flash before his "eyes", then sent him flying. It then escaped wreaking havoc upon Atlantis, which wasn't seen. Edd, Matt and Tom were then sentenced to death by oxygen for releasing the creature.