The resulting erosion at the base of a waterfall can be very dramatic, and cause the waterfall to recede. The area behind the waterfall is worn away, creating a hollow, cave-like structure called a rock shelter. Eventually, the rocky ledge (called the outcropping) may tumble down, sending boulders into the stream ...

A cave located behind a waterfall is often referred to as a "recessed cave" or a "grotto." Geologically, many of these are "erosional caves" formed through a process called retrograde erosion. This happens when the soft rock layer (like shale) underneath a harder caprock (like limestone) is eroded by the splashing water and mist, creating a hollow space behind the falling curtain of water. Some of the most famous examples include the "Cave of the Winds" behind Niagara Falls or the space behind Seljalandsfoss in Iceland. In some contexts, they are also called "rock shelters" if they are shallow. These spaces are often characterized by high humidity, mossy walls, and a unique acoustic environment where the roar of the waterfall is amplified. In 2026, many of these grottos are popular tourist sites, but they can be dangerous due to the risk of falling rocks (spalling) caused by the constant hydraulic pressure and freeze-thaw cycles of the water.