Complex sentence: They set out in a massive submarine, the Ulysses and as they approach the entrance to Atlantis, the Ulysses is attacked and destroyed by the Leviathan, a huge robotic guardian of Atlantis.
Split into simpler sentences: They set out in a massive submarine, the Ulysses. As they approach Atlantis, the Ulysses is attacked and destroyed by the Leviathan, a huge robotic defender of Atlantis.