Tunnelrunner

(Underdark, page 27, DnD 3.5)

[General]

You can move naturally in the cramped quarters of caves and tunnels.

Prerequisite

Chitine or grimlock

Benefit

You ignore the speed reduction for hampered movement when moving in a narrow space (an area smaller than but at least one-half of your space) or a low space (an area shorter than but at least one-half of your height). You also retain your Dexterity bonus to your Armor Class. If the space is both narrow and low, you function as if only one of those conditions applied.

Normal

In a narrow or low space, a creature without this feat is reduced to one-half normal speed and loses its Dexterity bonus to Armor Class. In a space that is both narrow and low, speed is reduced to one-quarter normal.