Technically not a desert but a landscape of red and gray rock, sculpted by ancient waterways, the Desierto de Tatacoa is one of Colombia's most unique ecosystems. It lies 4km east of Villavieja and can be explored on foot or by bicycle.
As you leave Villavieja you'll pass through Bosque del Cardón, a small cactus forest. Around 4km from Villavieja is El Cusco, where you'll find the Observatorio Astronómico de la Tatacoa.
A lookout point across the road from the observatory has impressive views, and is a fine place to watch the sunset. Below the lookout are the Laberintos del Cusco (Cusco Labyrinths), in a striking mazelike landscape of undulating red-rock formations that seem totally out of place in tropical Colombia.
Ventanas (the Windows), 4km past the observatory, is a lookout point, named for its commanding views of the desert, with rock formations carved into the shapes of animals. Another 5km takes you to Los Hoyos, where there is a swimming pool fed by a natural spring deep in a barren, gray valley.