Peña Cebollera, Cebollera Vieja or peak of the Three Provinces is a mountain located at the end of the main and western ranges of the Ayllón mountain range, in the Central System of the Iberian Peninsula.
With an altitude of 2128 m above sea level, it is the point of confluence of the limits of the Spanish provinces of Madrid, Guadalajara and Segovia, marked with a monolith at the top.
It rises above the port of Somosierra, a key passage in the communications of central Spain, and the town of the same name, the most immediate access point for hikers making the ascent.
To access Peña Cebollera from the Sierra del Rincón by mtb you must do so by first climbing the Collado del Mosquito