A stalagmite is a type of rock formation which appears in caves. It is considered a type of speleothem, or cave deposit: you may also be familiar with stalactites, another type of speleothem. Stalagmites grow very slowly over hundreds of years, and they can take on fantastical dimensions and shapes as they develop. Some very fine examples of stalagmites can be seen at Zhi Zen cave in China, Carlsbad Caverns in the United States, Grotta Ispinigoli in Italy, and Cueva San Martin Infierno, in Cuba.
A stalagmite forms when mineralized water drips from the ceiling of a cave to the floor. As the water slowly evaporates away, it leaves behind a small deposit of minerals, and this deposit will slowly build up in a cave where water drips in regular locations. Over time, the deposit will grow into a a roughly conical shape which points upward at the ceiling from the floor. Commonly, stalagmites are made from calcium carbonate, because they appear in limestone caves, although other minerals may appear in a stalagmite as well.
In cross-section, observers can clearly see the layers of deposited material which make up a stalagmite. In some cases, the composition of these layers may change in subtle ways, creating bands of texture and color which are invisible unless the stalagmite is cut open. These formations often occur near stalactites, mineralized formations which are caused by deposits on the ceiling of a cave.
Since the names for these two formations are very similar, some people have difficulty telling them apart. One thing which may help you remember is the shape. Stalactites have very regular elongated shapes, almost like icicles, with pointed ends. Stalagmites, on the other hand, have more irregular shapes, and they tend to be more broad, with flattened to rounded tops. Unlike stalactites, stalagmites are solid, but you have no way of knowing that without cutting the rock formation open, which is not recommended.
When you visit a cavern with speleothems, try not to touch them. These slowly growing rock formations are almost like living organisms, and the oils from your hands can disrupt the deposition of minerals, altering the shape of the resulting rock deposit. It is also a good idea to resist the temptation to remove stalagmites as mementos, since they take hundreds of years to form and by leaving them in place, you will ensure that future generations can enjoy them. Photography is encouraged at caves around the world if you want something to remember your visit by.