The Morning Glory Pool

The Morning Glory Pool in the Upper Geyser Basin of Yellowstone National Park in Wyoming. The pool's runoff channel is in the upper right of the picture.



And another photo from The Yellowstone Ecology Institute:



This popular thermal pool about a mile away from Ol;d Faithful is a rather large pool, which used to be completely deep blue but is in its current state due to pollution of the pool in the early 1900s as people tossed money and charms into the pool. The yellow fringe has grown as the pool has cooled and yellow colored thermophiles (bacteria) have increased pushing out the blue thermophiles. The pollution causes pools like this to clog and thus cool.