Hydrothermal karst speleogenesis. Yeah, I said it.

My geologist road dawgs talk like this all the time. For the rest of us, it is the process that creates a hot spring like this one.

The site is known as Korbous; it is located on Cap Bon, Tunisia. Here, hot, mineral-rich waters bubble up from the roots of the mountain and tumble back down into the Mediterranean. Such places are rare and therefore attract curious tourists, spa-seeking-bathers, and some serious algal growth (translation: slime).

This has been true for a long time. The Romans called the place Aquae Calideae Carpitanae and built baths here. One could go and sit in the 130 degree water after oiling down and scraping up. A first century inscription (now lost) was even mounted in the wall of one such building. It identified the benefactor (see source here).

"Decimus Laelius Balbus, son of Decimus, praetorian quaestor, had steam baths, a scraping room and a sun terrace built."

For Bible Lands Explorers, Korbous may evoke memories of Pamukkale ("Cotton Castle"), Turkey (adjacent to the ruins of ancient Hierapolis). As in Korbous, the mineral-laden water leaves a thick coat of white deposits on the rocks.