Travertine mounds form at the mouth of springs where CO2 degassing drives carbonate precipitation from water flowing from depth. Building of such mounds commonly involves the successive “stratigraphic” deposition of carbonate layers that precipitate from waters rising from depth along vertical to horizontal open fissures that are episodically sealed by radiating crystals. Much more intriguing structures can also be observed, such as widespread horizontal white veins of carbonate with vertical aragonite fibers, parallel or oblique to the “stratigraphic” travertines, which extend laterally over distances of several tens of meters and could represent up to 50% of the total volume of the travertine mound. Using highly precise U-Th dating, the growth direction of these horizontal veins is shown to be from top to bottom, and this fact clearly indicates that they developed within the mound over a period of ∼1000 yr for the vein analyzed. A vein growth mechanism is proposed that is able to uplift the rock above the vein thanks to the force of crystallization. The possible development of such structures in other places and the consequences of reverse growth direction when interpreting travertine data are discussed.