Around Old Faithful is the Upper Geyser Basin, which boasts the largest concentration of geysers in the world. Some of the better known are Castle Geyser (27m high), Grand Geyser (60m) and Riverside Geyser (24m). The variety of colours and shapes of these hot springs gives clues as to their creation and composition. The best known, Morning Glory Pool, is not as blue as it once was, as its spring has been blocked by objects thrown into the pool by visitors, causing it to cool.