americasgreatoutdoors:

If the conditions are just right, you can see a spectacular sight at Congaree National Park in South Carolina. These “rainbow pools” are not a sign of pollution, but a natural occurrence that can be caused by one of two things. Many times they are the result of decaying vegetation, especially cypress cones and needles, that release their natural oils. The other cause can be bacteria breaking down iron in the soil. Combined with very still waters, a few days without rain, and the correct angle of the light, and you can see shimmering colors floating on the water. Photo by National Park Service.