Crossing Texas and Mexico, the Big Bend region is high in biodiversity . It's a place so _ that if something doesn't bite, stick, or sting, it's probably a rock. You know you have arrived in the heart of the Chihuahuan Desert when it feels as if you have fallen off the edge of the earth and into the rabbit hole. Nothing is as it appears. Moths   are the size of birds. Are those twin pillars   of black rock (a landmark known as Mule Ear Peaks) ten miles (16 kilometers) away or fifty (80 kilometers)? Visibility   reaches more than a hundred miles on a clear day, and since there are few roads or buildings to use as milestones, distance is difficult to judge. This is a place where water runs uphill, where rainbows have to wait for rain. The line between myth   and reality is unclear. Stare long enough at the Chisos Mountains or the Sierra del Carmen, the two mountain ranges, known as sky islands, which lie on the land, and they rise and float above the plain. The vast Chihuahuan Desert is a land of no people. There is always the chance you'll die of thirst. The "You Can Die"possibilities are endless, and keep some visitors -- 350,000 a year to Big Bend National Park, built in 1944 -- from coming back. Those who do return are left to think of the remarkable courage of the brave few who have managed to survive in this terrible environment. What is the passage mainly about?
The natural wonders of the Chihuahuan Desert. Everything you see is not what it seems in the Chihuahuan Desert. The terrible environment of deserts in Texas and Mexico. A special place where none who go can return.