Needs review
Source: Underdark

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

Prerequisites:

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.