The Mariscal Mine is a ruin on the edge of nothing, which is exactly why it is worth the long dirt ride. In the southern part of Big Bend National Park the River Road runs west from the eastern side toward the Rio Grande, and in this corner of the desert you come across old adobe and stone buildings, rusted equipment, and the scarred slopes of Mariscal Mountain, where for decades people dug for mercury.
The ore was cinnabar, the red mineral from which quicksilver is refined, and mining here went on in the first half of the twentieth century. It was hard work in a harsh place: the heat is extreme, the water scarce, and the closest town was a long, rough journey away. The remains of the furnace and the workers' houses are still standing, and the whole site has a stillness that is difficult to explain until you stand in it. Mexico lies just across the river, and the Sierra del Carmen runs along the horizon on that side, pale and sharp in the afternoon light.
The riding is not for every bike. The River Road is a graded but rough dirt track, with sand, washboard, and stretches of rocky wash that become hard going when wet. An adventure bike with good tyres and a rider comfortable on gravel will manage, but a heavy tourer or road bike will suffer and possibly stop. Allow plenty of time, tell someone your route, and carry more water than seems sensible. Phone signal is poor to nonexistent. Check at a ranger station for road conditions before you set out, because summer storms can wash sections out for days.
Panther Junction, in the middle of the park, and the communities of Study Butte and Terlingua outside it, are where to fill the tank. Winter and early spring are the best seasons; summer is brutal, with temperatures that can top forty degrees. Look for roadrunners, javelinas, and the occasional coyote, and keep an eye on the sky in the evening, when the colours over the Chihuahuan Desert are remarkable.