Tufa is formed when calcium water bubbles up from below and combines with the carbonate in Mono lake. You wind up with calcium carbonate (er, limestone) towers growing under the lake.

When the lake level receded, due Jack Nicholson's Oscar-nominated portrayal of Mullholland in the movie Chinatown (or something like that), the tufas were exposed. As the lake fills back up, the tufas will be covered once again.