Tufa is essentially common limestone. What is uncommon about this limestone is the way it forms. Typically, underwater springs rich in calcium (the stuff in your bones) mix with lakewater rich in carbonates (the stuff in baking soda). As the calcium comes in contact with carbonates in the lake, a chemical reaction occurs resulting in calcium carbonate--limestone. The calcium carbonate precipitates (settles out of solution as a solid) around the spring, and over the course of decades to centuries, a tufa tower will grow. Tufa towers grow exclusively underwater, and some grow to heights of over 30 feet. The reason visitors see so much tufa around Mono Lake today is because the lake level fell dramatically after water diversions began in 1941 to supply the Los Angeles basin drinking water.
Today, after decades of effort, the water sources are now being reconnected from the mountains and the lake is rising slowly.