Arab merchants risked death each time they crossed the Sahara due to what?

Arab merchants tempted death every time they wanted to cross the Sahara desert because the latter is called the road of death by its extension. The "way of death" crosses more than 1500 miles. Which means that getting lost costs life.

Another reason why Arab merchants risked their lives every time they wanted to cross the desert was because only the nomads who inhabited the region knew the way to cross.