Geologists have found evidence that Pocket Basin is the crater left from a hydrothermal explosion (or steam explosion) some 10, 000 years ago. As the region warmed, glaciers started to melt, and a glacial ice dam broke, suddenly lowering the lake level behind it and reducing pressure below the former lake. The pressure drop caused the hot water in the underlying rocks to flash into steam, blasting large rock fragments to the surface and leaving a hole. This is called an explosion crater. The blasting happened because steam occupies about one thousand times the volume of the same amount of liquid water.
An Explosion Crater: Pocket Basin Photo Gallery
Click on Photos for Next An Explosion Crater: Pocket Basin Gallery Images
Since the steam could not escape to the atmosphere fast enough, it created a huge explosion. The result of this violent event was an oval depression about a mile (1. 6 km) across and a large assortment of interesting and very active mud pots over the ridge to the east of the former road. Sentinel Meadow Trail goes west just across the bridge near Ojo Caliente Spring. It's about a 2.2-mile (3. 5 km) walk to the Queens Laundry hot spring. On the way are several other springs and at least eight small geysers in the Sentinel Meadow Group of hot springs. Be aware that the whole area may be too wet for hiking before late July and that the trail provides no shade. Sentinel Meadow was named in 1872 for the high geyserite mounds or cones that seem to guard it like sentinels.
In the southwest corner of a large marshy area is the very hot spring called Queens Laundry. Here a two-room bathhouse was begun in 1881 by Superintendent Norris. The bathhouse was never completed, but the ruins are still there, now listed on the National Register of Historic Places. Another trail to Sentinel Meadow leaves the former road (now only for walking and bicycling) about a mile (1. 6 km) south of the bridge near Ojo Cali-ente. This trail forks to access the Fairy Meadows Group of hot springs to the northwest; on the southwest fork are the Fairy Creek Group, Imperial and Spray geysers, and Fairy Falls. Goose and Feather lakes are slightly farther along the main trail on the left. Goose has some rainbow and brook trout. In another mile or so, the main trail to Fairy Falls takes off to the west, described from the other end at mile 11.3/5.7.