The Elemental Plane of Earth is infinite earth. If you can't burrow, you're out of luck. If you can't breathe rocks, you're out of luck. Come well prepared if you think you'll stop by this plane.
There are a few places to go in this plane. The Great Dismal Delve is an immense series of caverns where the khan of the Dao resides in the Sevenfold Mazework. Visiting that place isn't recommended, though, since the dao are notorious slavers. Travelers here can find the Pale River and the Iron Crucible, vortices to the Planes of Water and Fire. There are also the hidden fortresses of wizards and trading outposts, magically tucked into pockets deep in the core of the plane. The plane of Earth's a popular place for emperors to banish their enemies, wizards to store their treasures, and even for paladins to hide evil artifacts.
The Chaggrin, Dao, Dharum suhn, Earth Elemental, Earth Genasi, earth+mephit, Earth weird, Earth whisper, Erdeen, Galeb Duhr, Horde, Khargha, Kryst, Pech, Rukarazyll, Sandling, Shad, Stonechild, Xaren, and Xorn are all native. Very few powers make their homes here - the most notable are Geb, Kabril+Ali+al-Sara+al+Zalazil (Great Khan of the Dao, the Fountain of Wealth, the Perfect Compass, Atamen of the Mountain's Roots, and so on) and Grumbar (an elemental lord).