Wikipedia:Today's second feature/January 1, 2006

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Aerial view of Grand Prismatic Spring

The Grand Prismatic Spring in Yellowstone National Park is the largest hot spring in the United States, and the third largest in the world, next to those in New Zealand.

The vivid colors in the spring are the result of pigmented bacteria in the microbial mats that grow around the edges creating the greens, yellows, oranges, and reds and refracted skylight creating the bright blue in the center of the pool.

Photo credit: Jim Peaco, National Park Service