On the low, sloping shore of a lake is an area of steaming pools, some brightly-colored, scattered over bare ground of mineral crusts and patchy vegetation. Heated water is rising to the surface here, emerging from the old volcanic rocks below. Clouds of warm, sulfurous water vapor drift over the site, downwind of the larger thermal pools.
Dimensions Many pools are tens of feet (many meters) across, but much smaller pools can also be found, commonly near large pool edges. Water in larger pools is many feet (several meters) deep, with visible throats extending down to even greater depths. The thermal pool area is a quarter to half mile (half kilometer to kilometer) across.
Thermal pool edges are flush to the ground, and clusters of pools occupy depressed areas of the shoreline slope. The view across the lake extends for miles, while evergreen forest rims the thermal pool area on all other sides.
In pools with water near boiling, the mineral deposits on the floors and rims are white, and water appears colorless or blue (reflecting sky color). At temperatures less than 167⁰ F, algae and bacterial mats grow with bright colors – yellow for highest temperature, then decreasing through orange, red, brown, and green. Colors other than green generally indicate scalding water. Where the water drains off over the ground surface, channels of more active (hotter) water flow, and shallow (cooler) puddles between, stand out in stark color contrasts.
Minerals solidify out of the water, depositing most heavily where the water surface meets the pool’s edge. This can build an unsupported ‘shelf’ of mineral crust out over pool waters (or even isolated ‘lily pads’). Although it may look stable, the shelf may not support the weight of a person.
Even at some distance from pool edges, it is possible the mineral crust caking the ground will give way underfoot, opening to shallow cavities of scalding water.
Water vapor clouds often hug the ground, completely obscuring view of anything more distant.
While the lake water is good for drinking and generally very chilly, vents of thermal, mineralized water do also emerge offshore, making local hot spots.
Year-round volcanic heat makes this a great place for winter camping or hunting. When most the lake is frozen over, the shore nearest the thermal pools will have open water for easy collection.
The shifting clouds of mist could hide/reveal unexpected visitors.
A local person has a big advantage here, including knowledge of especially useful/valuable mineral salts and bacteria.
People could not survive in the pools, but what might? This could inspire science fiction, horror, fantasy, and other genres.
To avoid the danger of mineral crust false-ground collapse (with scalding water below), canny travelers would follow foot trails of other people or large animals, or limit travel to areas of scrub grass. If forced to cross mineral crust areas near pool edges, they might use a means of broadly distributing weight (a movable board, jury-rigged snowshoes).
A character challenge: how to get a clue, treasure, message, mineral sample out of a pool or from the delicate mineral shelf.
Sensory experience of the thermal pools:
The smell of sulfur would inform wise travelers approaching this area to be wary.
Shifting breezes would bring warm, moist air chased by cooler, dry air.
Colors of the ground are unexpectedly intense here.
Occasionally, travelers might hear the sound of bursting bubbles. The mineral crust would crunch underfoot.
West Thumb thermal area and nearby geyser basins, Yellowstone National Park, northwestern Wyoming.