There is a piece of Yellowstone Lake known as the "West Thumb". As there is no North, South, or East Thumb I'm not sure why it has earned that distinction. It is presumed that the thumb is a separate crater from a volcanic explosion and the area is still geologically active.
Yellowstone Lake as shown on Google Maps.
The West Thumb Geyser Basin contains an array of geysers, pools, and "paint pots" some of which are adjacent to and even in the lake.
Thumb Paint Pots
Stories say that in days past on the lake a fisherman could fish from the shore and pop the catch right into the cone of the geyser to cook it on the spot. There are also tales of what could happen to a fisherman if he straddled the cone.
Yes, I know it's very, very blue.