The Trans-Pecos, as originally defined in 1887 by the Texas geologist Robert T. Hill, is the portion of Texas that lies west of the Pecos River. The term is considered synonymous with "Far West Texas", a subdivision of West Texas. The Trans-Pecos is part of the Chihuahuan Desert, the largest desert in North America. It is the most mountainous and arid portion of the state, and most of its area (outside the city of El Paso) is vast and sparsely populated, comprising seven of the ten largest counties by area in Texas. The area is well known for the natural beauty of the Big Bend, and has one of the highest proportions of publicly owned land in the continental US.