Inch by inch, row by row, gonna make this geyser grow! Castle Geyser is a prominent landmark in the Upper Geyser Basin with a cone that is thousands of years old. Hot water passes through silica-rich volcanic rock deep underground. As the water cools on the surface, it leaves behind the mineral geyserite, also known as siliceous sinter. Standing over 12 feet (3.66 m) tall, Castle’s large structure is possibly due to trees growing in the area years prior that became coated in sinter and allowed the height to build up. Layers of geyserite cascade in step-like terraces from the geyser vent and continue to cover the surrounding area. Image: Water and steam erupts from a large gray geyser cone with two rainbows on the side.