Nessus

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

The blasted plain of Nessus floats in a swirling, crimson void. Its roughly ovoid, jagged-edged land mass extends for approximately 2,500 miles from east to west and about 1,100 miles from its northernmost to southernmost points.

The absolute flatness of Nessus allows a viewer to see for miles in all directions. Devoid of trees, structures, and changes in elevation, its desolate landscape provides little or no cover. Beings moving across its surface can be seen for hours before they finally close to ranged weapon distance.

A network of crisscrossing gouges, gorges, and canyons scars the dry and rocky plain. The largest of these crevasses are miles across and dozens of miles deep. The layer's millions of inhabitants dwell within these canyons, either in caves carved out of the walls, or on the crevasse floors. Although the surface of Nessus is finite, its twisting canyons and tunnels contain an effectively infinite amount of space.

The canyons intersect confusingly across the surface of Nessus. In a few places, they weave around each other, sculpting towering mesas. To reach a specific destination, explorers must be able to tell one gorge from another.

In keeping with the layer's utter hostility to travelers, the crevasses are completely unmarked. Maps of the surface have been cobbled together from sketches created by generations of suicidally brave cartographers. But even when accurate, these maps are difficult to match with conditions on the ground. Finding a particular point on Nessus's surface requires a successful DC 30 Knowledge (geography) check. A map can grant a +1 to +4 bonus on this check, depending on its accuracy.

Few bridges span the canyons, and those that do exist are either heavily guarded or trapped. Any seemingly unguarded bridge is likely to hurl travelers into the gorge below if the correct password is not uttered when they reach the midway point. Without exception, the gorges of Nessus are deeper than 200 feet, so anyone plummeting into one takes 20d6 points of damage.

Flightless devils are expected to remain in their home crevasses. In cases of emergency, though, they can travel up and down the walls either by holding tight to treacherous handholds (Climb DC 20) or by using the fraying rope ladders (Climb DC 10). Inhabitants with poor climbing skills tend to stick close to their caves or hovels, venturing forth only when their lives depend on it. Indeed, devils plunge to their deaths from the canyon walls on an infrequent but regular basis.

From Manual of the Planes (3e):

The ninth layer is the deepest realm of the Nine Hells, a plain shattered by rifts deeper than the deepest ocean trench. Many of the ravines and canyons here reach thousands of miles into undifferentiated, dead stone. Most of the trenches seem natural, but some appear as if they were cut or blasted into the land. Rumor has it that an offshoot of the Styx flows here and there, dropping into trenches and trickling its way across the layer. Few know how to reach this tributary, if it exists at all.