Complex sentence: Doctor Omega and his two companions travel to Mars, where they first land in one of the Martian seas, where they perform some underwater exploration during which they encounter phosphorescent fish and aggressive reptilian mermen.
Split into simpler sentences: Doctor Omega and his two companions travel to Mars. They first land in one of the Martian seas, and do some underwater exploration, during which they encounter phosphorescent fish and aggressive reptilian mermen.