Along the Biscuit Basin Trail you'll find this large clear blue pool with a solid sinter rim level with the ground surface. The pool's edges have shallow sinter shelves that contain yellow and orange thermophiles. The Sapphire Pool was once an active geysering pool before the 1959 earthquake. Following the earthquake they filled with muddied water and began to have more significant eruptions. In 1971 the pool's color cleared and all activity stopped. The minor geysering before 1959 has not returned and the pool has not had an eruption since 1991, indicating a change to the plumbing system due to the 1959 earthquake. Sapphire Pool has an average temperature of 159.8°F (71°C); this is still one of the most colorful pools in the Biscuit Basin with ease along the boardwalk. You will find the Sapphire Pool Along the Biscuit Basin Trail. The parking area is always full here, but if you time it right (when people are at Old Faithful), you may find a spot!