4. Grand Prismatic Spring.
The Midway Geyser Basin’s Grand Prismatic Spring is the largest hot spring in the United States, approximately 370 feet in size and around 121 feet deep. But its rainbow waters are what really make it fascinating. This natural wonder is one of the park’s most photographed sights, so expect crowds when you visit. You may find fewer visitors if you arrive early, but fog is common until late in the morning on cooler days, so travelers recommend arriving around 10 a.m.