Today’s Wonder of the Day was inspired by Mariah from , . Mariah Wonders, “what is the difference between stalagmites and stalactites?” Thanks for WONDERing with us, Mariah!

As you walk around outside, do you ever think about what's under your feet? We're not talking about what's directly under your feet, such as grass, sidewalk, or pavement. We mean deep underground.

Depending upon where you live, there could be a whole other world filled with WONDER and beauty beneath your feet. What are we talking about? Caves, of course!

In areas with large deposits of limestone underground, you're likely to find caves. Scientists call these karst areas. What is it about limestone that leads to cave formation? Limestone is easily dissolved by carbonic acid.

As rain falls through the sky and groundwater filters through the soil, the water absorbs carbon dioxide from the air and the soil. This transforms the water into a weak form of carbonic acid.

When the carbonic acid reaches limestone underground, it dissolves small amounts of calcium contained in the limestone rock. This leaves gaps and holes in the rock that, over long periods of time, develop into larger open spaces we call caves.

If you've ever been in a cave, you've probably noticed some very interesting formations within the cave. These unique cave decorations are created by the reversal of the same processes that form the caves in the first place.

As the watery carbonic acid solution continues to seep into cave openings that are now filled with air, dissolved carbon dioxide gets released into the air. When that occurs, the remaining water can't hold as much dissolved calcium. The excess calcium gets deposited on the ceilings, floors, and walls of the cave, creating unique formations known as speleothems.

The word "speleothem" comes from the Greek words spelaion, meaning cave, and thema, meaning deposit. Most of the calcium deposits occur in the crystallized form of the mineral calcite. Slight variations in the process can result in a wide variety of different types of speleothems.

Common speleothems include dripstones (such as stalactites, stalagmites, straws, columns, or pillars), flowstones (such as shawls, curtains, draperies, or "cave bacon"), pore deposits (such as helictites and cave corals), and pool deposits (such as rimstone, dogtooth spar, cave pearls, shelftstone, or lily pads). Speleothems can vary greatly in size, composition, and color throughout a cave system.

Two of the most common and popular types of speleothems are stalactites and stalagmites. Stalactites grow downward from cave ceilings. They begin to form as straws but eventually grow into stalactites as the straw form becomes blocked with calcite. As water continues to drip down the stalactite, leaving tiny bits of calcite, the stalactite continues to thicken and grow over time.

When those drops of water fall from the ceiling or stalactites and deposit calcite on the floor, stalagmites form. Stalagmites are solid dripstones that grow upwards from cave floors. As water drips onto the stalagmite from above, bits of calcite get left behind and the stalagmite continues to grow and thicken over time.

A common way to remember the difference between to two formations is to say that stalactites cling tightly to the ceiling, while stalagmites might reach the top if they grow large enough.

Wonder What's Next?

Tomorrow's Wonder of the Day will have you traveling all over the world...without ever leaving home!