General terms:
* Geothermal spring: This is a broad term that refers to any natural spring heated by geothermal activity. It could be a pool, a stream, or a geyser.
* Hot spring: This is another common term, often used interchangeably with "geothermal spring."
* Thermal pool: This term emphasizes the water temperature and is often used for pools specifically designed for swimming or relaxation.
More specific terms:
* Geothermal pool: This term is often used to distinguish a naturally heated pool from one that is artificially heated.
* Underground hot spring: This emphasizes the location of the spring.
* Cave pool: If the pool is located in a cave, this term is appropriate.
Other possibilities:
* Natural hot tub: This term is used for smaller, more intimate hot springs.
* Geyser pool: This term is used for pools that are fed by geysers.
Ultimately, the best term to use depends on the specific context and the characteristics of the pool.
