Complex sentence: They set out in the "Ulysses", a massive submarine, but are soon attacked by the monstrous Leviathan, a robotic lobster-like creature that guards Atlantis' entrance.
Split into simpler sentences: They set out in the "Ulysses", a massive submarine. During the journey, they are attacked by the monstrous Leviathan, a robotic lobster-like creature that guards Atlantis' entrance.