A desert is a dry place that gets very little rain. Some deserts are hot, but others have cold winters or are cold all year. The Great Basin is a cold desert that gets less than 10 inches of rain a year, mostly as winter snow. Sand dunes cover only about 10 percent of the world's deserts; others are dry expanses of rock, salt flats or even ice sheets. Let's start with the plants that grow in these dry lands.

Big sagebrush root systems can extend as much as 90 feet in circumference. A mature pinyon pine in the high desert has a root system at least as large as the tree above ground, with taproots reaching 40 feet or more down. A saguaro's roots work in a different way. They spread out just a few inches under the ground, so they can catch water from a quick rain. A saguaro usually starts to grow its first arms when it is between 60 and 75 years old.

Kangaroo rats live in deep underground burrows that they seal off to keep out midday heat and to recycle the moisture from their own breath. A kangaroo rat also does not need to drink: it gets its water from the seeds it eats. Gila monsters spend more than 90 percent of their lives below ground, and the Gila monster is a venomous lizard that lives in the deserts of the American Southwest and northwestern Mexico. A desert tortoise can store up to 40 percent of its body weight in water inside its bladder.

People have also crossed deserts for a very long time. Camels were domesticated more than 3,000 years ago, and people still use them to cross dry lands. Trade across the Sahara increased greatly after people domesticated the camel. From the 8th century on, Berber caravans across North Africa brought great wealth to those who controlled the trade links and centers. In the desert heat, a traveler should drink plenty of water and stay in the shade during the hottest part of the day.

An oasis is a place made fertile by fresh water in an otherwise dry region, where natural springs or underground water irrigate the land. Whoever controls an oasis also controls the trade along the route, which made oases important to leaders. The ancestral Puebloans built villages around springs in the Colorado Plateau desert.

Words to know

• caravans: traveling groups
• oasis: watered place
• taproots: main roots
• domesticated: tamed
• venomous: having poison