Devils Hole, a detached unit of Death Valley National Park in Nevada, maintains a remarkably constant water temperature of approximately 93°F (34°C) year-round. This extreme temperature is not caused by sunlight, as the water is located at the bottom of a deep limestone cavern, but rather by geothermal heating from the Earth's crust. This "hot" environment is one of the most stressful habitats on Earth for fish, yet it is the only natural home of the critically endangered Devils Hole Pupfish. The water is not just hot; it is also very low in dissolved oxygen, making it a "biological island" where life exists at the very edge of its physiological limits. The temperature is so stable because the pool is part of a massive underground aquifer system that spans hundreds of miles; the sheer volume of water acts as a thermal buffer. For a human, 93°F feels like a lukewarm bath, but for the pupfish, it is a constant struggle for survival, especially during the summer when the shallow "spawning shelf" can heat up even further, occasionally reaching temperatures that can be lethal to their eggs.