El Leoncito is brutally dry, taking in only about 200 millimeters of rain a year, and that aridity is its gift. Clear, cloudless nights on roughly 300 evenings a year make it one of the finest windows onto the cosmos anywhere on the planet. The park sits high in the Andean highlands, where summer days can climb to 35 degrees Celsius and winter nights plunge to minus 10, and where the thin, stable air holds the stars steady rather than letting them shimmer. Bring a jacket if you come to look; the mountaintops turn bitterly cold after dark. The reward is a sky most people will never otherwise witness, undimmed by the glow of any town.

Two observatories share the park, and both welcome visitors to look through their instruments. The larger, the El Leoncito Astronomical Complex known as CASLEO, was established in 1983 as a joint venture of Argentina's national research council and three universities. Inside its white dome stands the Jorge Sahade telescope, a 2.15-meter reflector built by the firm Boller and Chivens, installed in 1984 and running regular observations since 1987. It remains the largest optical telescope in Argentina. The second, the smaller CESCO complex, also opens its domes to night-time visitors who arrive after dark. Reservations help, and schedules shift with the seasons and the moon. There is something fitting about a national park whose star attraction is, quite literally, the stars, where the protected resource is not a waterfall or a peak but the darkness itself.

Below the observatories spreads the Pampa El Leoncito, a vast, bone-flat plain that was once the bed of an ancient lake, roughly 12 kilometers long and 5 wide, sitting near 1,900 meters above sea level. Today it is a radiantly white expanse where almost nothing grows and the wind blows hard and steady across the open ground, gusts after midafternoon climbing toward 80 kilometers an hour. That combination makes it one of the world's great spots for land sailing, called carrovelismo here, the sport of skimming across the flats in a light, three-wheeled cart rigged with a sail. The afternoon wind does the work, and the cart can travel far faster than the air that pushes it; the best pilots clock speeds past 100 and even 120 kilometers an hour on the smooth, hard surface. The prime season runs from October into late February, when the winds are strongest. From a distance the sails look like tiny bright triangles racing across a frozen white sea.

For all its harshness, the park holds tenacious life. Guanacos, pumas, and foxes range the highlands, and the rare lesser rhea, a smaller cousin of the great flightless bird, picks across the steppe. In the park's small streams swims a catfish found nowhere else, isolated, biologists believe, when its streams stopped reaching the San Juan River, leaving its ancestors stranded to adapt to ever-harsher conditions. There is even a thread of human history running through it: a stretch of the old Inca road system crosses the park, a reminder that people moved through these heights long before telescopes or sail-carts arrived. Entry is free, the nearest town is little Barreal some 35 kilometers off, and the silence is total.