From the introduction:
Pellucidar is a land 500 miles below the surface of the earth. A miniature sun suspended at the center of the hollow sphere always shines; there is no night on Pellucidar. The miniature sun never changes in brightness, and never sets; so with no night or seasonal progression, the natives have little concept of time. The world is additionally more vast than our own. Our oceans and their continents and, correspondingly, our continents are their oceans. Time is elastic, and months for one person can seem like a few hours for another. Because time flows so differently at the Earth's Core, long-second watches no longer function. And because of the permanent noon-high sun, and because of the unnatural lack of a horizon, all PC's lost 1d4 Agility and Endurance for 1d4 days upon first reaching Pellucidar.