Hot and boiling springs in Kenya and New Zealand that are emitting water with temperatures more than 90 °C are commonly characterized by a complex array of CaCO3 and SiO2 precipitates that have been formed through abiogenic and biogenic processes. Thermophilic bacteria are the only microbes that can survive in the boiling water that is discharged into pools around the spring orifice. Analysis of modern substrates from various springs in the Kenya Rift Valley and the Taupo Volcanic Zone in New Zealand shows that they are inhabited by a diverse array of coccoid and filamentous bacteria. In some areas these bacteria produce copious amounts of mucus that coat the substrates. Although the coccoid and filamentous bacteria provide substrates for CaCO3 and SiO2 precipitation, the microbes do not seem to have any direct influence on the morphology of the precipitates that are produced. Conversely, the mucus found in these hot spring pools selectively takes up elements such as Si, Mg, Al, and Fe, but is not calcified. In many cases the elements that are selectively fixed by the mucus are only present in very low concentrations in the spring water. In one of the Waikite springs in New Zealand, the mucus plays an active role in the formation of the calcite deposits by providing a template for crystal nucleation and binding the small calcite crystals to the substrate. The latter process is especially important because the flowing waters of the spring could easily transport the grains if they were not bound to the substrate.