1) The characters explore an underground sea filled with prehistoric animals like alligators over 100 feet long and huge snakes.
2) They encounter a dangerous electrical storm while sailing across the underground sea.
3) They discover a lost world containing mammoths and human-like creatures dressed in animal skins on the shore of the underground sea.
4) They use gunpowder to blast open a rock blocking a tunnel, hoping it will lead them to the center of the Earth.