Plop.

Plop.

Water drips from stalactites hanging from cave ceilings.

Each successive drop adds a thin layer of minerals to a growing stalagmite below. These remarkable structures are found on the floors of caves all over the world. Although they grow only a fraction of an inch each year, they serve as record keepers of past environmental conditions, such as droughts.

But while there has been a lot of research focused on how stalactites form, relatively little has been done on stalagmites.

Simulations of their growth tend to assume that water droplets fall straight down, resulting in candlestick shapes. But that is not what is observed in nature. Stalagmites are found with cone, dome and even “pile-of-plates” formations, which resemble stacked dishes.

New research published last week in Proceedings of the Royal Society A sheds light on why stalagmites have diverse shapes. The study presented a physics-based model that shows that stalagmite forms are dictated by how far water droplets fall from their stalactite of origin. Drips that fall farther tend to result in wider stalagmites. These findings can be used to reconstruct a cave’s geometry over time, the research team suggests.