The terrestrial limestones forming at the emergence of thermal springs show a variety of unusual depositional facies. The specific lithological and petrological features of these deposits have few counterparts in the marine, and continental, karst-related carbonates, but they are typical of the epigean limestone that has been quarried since antiquity in the surroundings of Tivoli (Rome) under the name of travertine, where it is still forming in hydro-geothermal fields linked to extensional tectonics. The physicochemical, hydrodynamic and geological conditions specific to the Tivoli thermal spring system imply hypersaturated alkaline–sulphate, warm to hot waters, upwelling from springs fed through open fractures/faults in extensional and/or volcanic regimes. These features, together with the hydrodynamic behaviour of the water flows running from the vents, control the petrogenetic features of the travertine, a well-bedded, mostly finely laminated, porous but quite compact limestone. The results of a detailed comparative petrological analysis carried out on the lithofacies of travertine limestones, and of those observed during formation within numerous active thermal spring fields, provide the elements required for an exhaustive textural classification of the travertine lithofacies, which has not yet been described systematically. According to the genetic processes and fabrics, the thermal deposits that originate from such hypersaturated alkaline–sulphate, warm to hot waters, can be subdivided into: abiotic crystalline crusts, microbially mediated crusts (microbialites) and granular deposits mostly represented by small accumulations of lime-mudstone. Some of the granular deposits and the microbialites are only partially comparable with analogous sediments forming on tidal flats/sabkhas or other continental sites; however, the facies association of crystalline crusts and laminar curled microbialites has no counterpart in the marine realm. The widespread presence of thermophile bacteria and sulphobacteria, and the general absence of autochthonous eukaryote organisms, unable to live in poisonous sulphate waters, are also undeniable evidence of their thermal origin.