[2] It takes its name from nine small, deep lakes located along the eastern escarpment of the Pecos River valley.

Its name stems from the water's dark color, caused by the steep sides of the cenote and algae growth within the lake.

When the water is very high the strip of land is covered, and the two nearly circular lakes join and take the shape of a figure eight.

The Bottomless Lakes occur at the base of an escarpment formed by the gypsum-rich Seven Rivers Formation.

Ground water in the underlying San Andres artisian aquifer rises along faults at the escarpment and dissolves the gypsum thereby creating sinkholes or cenotes.