Mono Lake Tufa

The tufa towers poking out of Mono Lake are made of calcium carbonate, not unlike the formations that grow among hot springs and geysers.

This is the place where some NASA biologists gathered bacteria they claimed could happily substitute arsenic in place of the regular phosphorus needed for cellular functions. A life form based on poison? Sensational!

Sadly, it was thoroughly debunked; see Slate's accessible summary, or the technical refutation by UBC biologist Rosie Redfield. The methods were sloppy and the claims unsubstantiated. It is unlikely that any of the bacteria could live without any phosphorus.