This is called the "Fishing Cone" on Yellowstone Lake. According to legend, the name derives from fishermen who would drag fish, while still on the line, into the hot springs to cook.
Here is one early explorer's account: "Several springs were in solid rock, within a few feet of the Lakeshore. Some of them extended far out underneath the lake; with which, however, they had no connection. The lake water was quite cold, and that of the springs exceedingly hot. They were remarkably clear, and the eye could penetrate 100 feet into their depths, which to the human vision appeared bottomless. A gentleman was fishing from one of the narrow isthmuses, or shelves of rock, which divided one of these hot springs from the lake, when, in swinging a trout ashore, it accidentally got off the hook and fell into the spring. For a moment it darted about with wonderful rapidity, as if seeking an outlet. Then it came to the top, dead, and literally boiled. It died within a minute of the time it fell into the spring." - William Trumbell, 1881