Hot springs in the Upper Geyser Basin carry dissolved silicates to the surface from the volcanic rock below. When the hot water cools and evaporates, it deposits the silica, forming geyserite or sinter. Doublet Pool creates a particularly beautiful scalloped sinter formation all around its edge. About every half-hour Doublet rises about four inches and overflows off Geyser Hill, the process slowly building its lacy collar of rock. When the pool is full, the water column bounces up and down and makes a deep thumping sound as tons of water bang against the sides and bottom of its crater. You can feel the boardwalk shake every time it thumps.