The topographic features known as abyssal hills characterize >30% of the ocean floor, and yet their origin has been the subject of vigorous debate for over 40 years. Submersible-based investigations show that Pacific abyssal hills are created on the flanks of the East Pacific Rise as horsts and grabens which lengthen with time; Hills are bounded on one side by ridge-facing scarps produced by normal faulting, and on the other by more gentle slopes produced by volcanic growth faulting.