Grand Prismatic Spring is the crown jewel of Yellowstone National Park's thermal features and the largest hot spring in the United States, a vast, steaming pool of impossibly vivid color in the Midway Geyser Basin. Measuring 200 to 330 feet across and more than 121 feet deep, it rings a core of deep sapphire-blue water with concentric bands of orange, yellow and green, colors produced by mats of heat-loving thermophilic bacteria living in the progressively cooler water toward the edges. Seen from above, it looks like a giant's eye staring up out of the earth.

There are two ways to take it in, and both are worth doing. The boardwalk at Midway Geyser Basin crosses the Firehole River and snakes right past the spring, letting you stand at its steaming edge, though at ground level the sheer size and the drifting steam can make it hard to appreciate the full rainbow. For the iconic overhead view, the nearby Grand Prismatic Overlook trail climbs a hillside to a platform that reveals the whole spring and its color bands at once, which is the shot everyone comes for.

This is a look-only wonder, and the safety rules are literally a matter of life and death. It is illegal and extremely dangerous to leave the boardwalk: scalding water underlies the thin, breakable crust around the spring, the water itself can cause severe or fatal burns, and more than 20 people have died after entering or falling into Yellowstone's hot springs. The National Park Service is emphatic that visitors must stay on the boardwalks and designated trails at all times, never run or push, and keep close hold of children. There are no railings on much of the boardwalk.

The basin is busy, and parking at the Midway lot fills fast through the day, so early morning or late afternoon is best for both crowds and light; steam is heaviest on cold mornings, which can obscure the colors, so a warm, still midday actually shows the spring off better than a frosty dawn. The overlook trail adds a bit of uphill but rewards it enormously.

Grand Prismatic sits along the Grand Loop Road between Old Faithful and Madison, an easy stop on any Yellowstone itinerary. It is one of the most photographed natural features on earth, and standing beside its steaming, jewel-toned water, or looking down on the whole rainbow from the overlook, is a genuinely unforgettable Yellowstone experience.