Black Sand Basin

Black Sand Basin is a compact hydrothermal basin in Yellowstone's Upper Geyser Basin, about a mile northwest of Old Faithful along the main road. Its short boardwalk links colorful springs and small geysers, making it a good add-on if you want more than one pool without taking on a long walk. Choose it for variety in a short visit; prioritize Grand Prismatic if you have time for only one large thermal basin.

Emerald Pool, Rainbow Pool, Sunset Lake and Cliff Geyser are the basin's headline stops. Sunset Lake is a geyser, despite the name; its eruptions are infrequent and heavy steam can hide them. Cliff Geyser sits at the edge of Iron Spring Creek and can send water up to 40 feet. If it is quiet during your visit, the creek and the colorful edges of the pools are still worth a look.

Rain and snow seep down through cracks, heat underground and rise again through Yellowstone's natural plumbing. Microscopic heat-loving organisms gather in visible mats around the springs: yellow and colorless communities favor the hottest water, while orange, brown and green show up where it is cooler. From the boardwalk, frame the blue-green water with its orange-yellow margins to show the contrast.

The basin takes its name from black sand made from volcanic glass called obsidian. For a time in the late 1800s, it was known as the Emerald Group after Emerald Pool. The two names point to different things that define the basin: its dark volcanic sand and the spring whose color drew early attention.

The basin is also reachable by a one-mile walk from Daisy Geyser. Visitors describe the short boardwalk as starting beside the parking area. It is marked wheelchair-accessible, though a few boardwalk sections are steep. Stay on boardwalks and designated paths: the hydrothermal water can cause severe burns, and children need close supervision.