As lava flows above ground, the exposed lava surface cools down and forms a hard crust. The hot lava beneath the crust continues to flow downslope until it drains out, leaving empty caves behind. The caves become accessible when their fragile ceilings collapse, normally within 10,000 years after they form. Jordan’s lava caves were formed at least 400,000 years ago; they are among the oldest in the world. Stone tools found around some caves suggest that they were used by humans in prehistoric times.
At least seven lava caves are found in the Basalt Desert. The longest is Beer al-Hammam, 445 meters long; it is located at the GPS coordinates: 32.1326°, 36.8233°. The most accessible is the Azzam Cave at the GPS coordinates: 32.2850°, 36.6099°.