Avernus

From Fiendish Codex II: Tyrants of the Nine Hells (3.5e):

The entryway to Baator seethes with danger. Devil armies muster on Avernus's stony wasteland, and its primary waterway, the River Styx, robs intelligent creatures of their identities within moments of exposure. Other pools and rivers run thick with the blood shed in a million battles, ready to claim the lives of any who slip from their rocky, gore-slicked banks. Fireballs careen through the atmosphere, randomly incinerating those who are not immune to their effects. Toothy mountains loom on every side, offering fatal obstacles to those unwise enough to scale them,  and sharp edges of crystalline rock tear at flesh and clothing.

Blood and viscera coat the entire surface of Avernus. That lump underfoot might be a rock, or it could be a stray bit of bone or horn ripped from the body of a devil or a demon. Although Bel, archduke and chief general of the Nine Hells, does his best to fight his battles against demonkind in enemy territory, his layer is invaded by tanar'ri on a regular basis. He has pushed them back after every incursion, but only after his territory has been spattered afresh with gore.

From Manual of the Planes (3e):

The first layer of Baator is a wasteland of charred, rubble-strewn plains, with mountains and talus-sided foothills breaking up the monotony. Legions of mail-clad devils are on perpetual watch here, always assembling for some new cross-planar Blood War sortie. Blood-red light suffuses the air, and fiery balls flit randomly across the sky, sometimes detonating to terrible effect. Travelers not protected by artificial structures or caves are 10% likely per day to find themselves at ground center of a fireball dealing 6d6 points of fire damage (as if cast by a 6th-level spellcaster).

Runnels of blood flow in streams across Avernus, finally joining the River Styx. The origin of the blood is unknown, though the devils say it is likely the blood of all Avernus's past victims.