The world is hollow, you say? An absurd idea! But at one time many believed this to be the case. Some still do. Edmund Halley is most famous for the comet named after him. He also thought the earth was hollow, a theory he proposed in 1692. The biggest push for the idea, though, came from John Cleves Symmes, who published a pamphlet in 1818 describing how access to the interior could be achieved by traveling through openings at both the north and south poles. Symmes even petitioned the U.S. Congress to fund an expedition to "Symmes Hole!" Sadly, the sticks-in-the-mud in Congress weren't interested.
Pellucidar was first discovered by David Innes and Abner Perry, who built an "iron mole" to reach mineral wealth deep in the Earth. Their journey ended abruptly when they emerged into the inner world of Pellucidar. The adventures of Innes and Perry were chronicled, starting in 1915, in Edgar Rice Burrough's series of novels about it, beginning with To the Earth's Core.
Pellucidar is a world without time, without night, and without mercy. Dinosaurs and prehistoric mammals of all sorts roam the interior of our Earth. Humanity is a few scattered tribes of primitive men and women, who are under the control of the beastly Mahar and their savage Sagoth enforcers.
But Pellucidar is not the only variant of the Hollow Earth theory in popular fiction or gaming. DC Comics once had Skartaris, a lost world accessible only by the polar regions, in which Travis Morgan became Warlord. TSR had a Hollow World RPG. Back to Victorian Science Fiction, the idea is in use by When the Navy Walks, in the world of Earthin.
Earthin is a strange place inhabited by fantastic beasts and humans of many different cultures. These cultures evolved apart from their surface cousins, and are therefore noticeably altered from them. I can't go into a great deal of detail about this mysterious place... yet.