Western Kentucky's Mammoth Cave is the longest cave system on Earth, with 412 miles known and mapped as of fall 2017. The name does not refer to the early discovery of fossil mastodon or mammoth bones here. Rather, the name refers to the immense size of many rooms and passages.

Shown here is a dissolutionally-grooved limestone wall in Main Cave, the principal trunk passage in the Mammoth Cave Ridge portion of the system. The rocks are marine limestones of the upper Ste. Genevieve Limestone (upper Middle Mississippian).

Terms used for dissolutional grooves formed on limestone rock walls in the vadose zone of caves include "flutes" and "rills". Flutes occur on vertical walls of domepits/vertical shafts. Rills occur on slanted limestone surfaces. The rills shown above are on very steeply slanted, nearly vertical surfaces. Rills form by dissolution from slightly acidic water draining from solution pockets after high flow events.