Leviathan

The Leviathan is a giant Atlantean artifact that was left near the underwater entrance to Atlantis so that it could protect the city from intruders. Being longer than a submarine, the Leviathan is a mechanical monstrosity that closely resembles a lobster. According to Milo Thatch, the Leviathan's lair has ships from every era, meaning that the giant has killed many men throughout the centuries. It is also capable of flight as seen just before the destruction of Atlantis.

Description
The Shepherd's Journal describes the Leviathan as a sea serpent; while the Bible describes it as a beast that breathes fire from its mouth. Milo Thatch thought this was merely a ruse, however, and thought that it was a statue. Unbeknownst to him, the Leviathan was, in fact, a real machination that subsequently attacked the Ulysses.

History
In Atlantis: The Lost Empire, the Leviathan, or another of its kind, is seen being swallowed by a gigantic tsunami accidently/purposefully generated by the King of Atlantis. It is then charged to guard the entrance to Atlantis.

Nearly 8,000 years later, it attacks the submarine Ulysses for trespassing on its territory. The submarine is destroyed, but a few crew members survive.

In Atlantis: Milo's Return, the Leviathan is known to have investigated the return of Team Atlantis, who returned using the original route instead of using the volcano for some unknown reason, as they would not have encountered the being otherwise. It apparently sensed the Atlantean crystal necklaces the team wore that identified them as Friends of Atlantis and allowed them to pass. Later on, it is mentioned in passing as a possible suspect regarding Kraken attacks, although the Kraken was neither Atlantean nor mechanical.

It is unknown what became of the creature after the raising of Atlantis, but it is presumed that it continues to guard the (no longer needed) entrance.

Abilities and Weaknesses
Being a giant, the Leviathan can easily crush any enemies. It can also shoot lasers from its mouth, which can easily destroy a small ship in one hit. However, due to its size, any ship that manages to elude its capture can get into the entrance to Atlantis, where it is too big to fit.

In the original treatment, Milo figures a way to destroy the Leviathan when in a Sub Pod with Helga Sinclair. However, this concept was dropped early in pre-production. All that remains of this are one or two storyboards and an animation test that also contained an early design idea for Helga.