During an expedition to Iceland, professor Trevor Anderson (Brendan Fraser), his nephew Sean (Josh Hutcherson) and their guide, Hannah (Anita Briem), get trapped in a cavern. As they follow their only escape route deeper and deeper below the Earth's surface, they pass through strange places and encounter incredible creatures, including dinosaurs.

The story starts with a comet called Gallia, that touches the Earth in its flight and collects a few small chunks of it. The disaster occurs on January 1 of the year 188x in the area around Gibraltar.

During the American Civil War, five Northern risoners of war escape during the Siege of Richmond, Virginia, by hijacking a hydrogen-filled observation balloon.

The story opens some time after the end of the American Civil War. The Baltimore Gun Club, a society dedicated to the design of weapons of all kinds (especially cannons), comes together when Impey Barbicane, its president, calls them to support his latest idea.