ad-Dahna Desert

It is also considered the geographical margin separating Al-Ahsa Province from Najd.

[2] Ad-Dahna desert is formed of high sand dunes spreading horizontally which are called veins (Arabic: عُرُوْق, romanized: ʿUrūq), mostly red in color since it contains Iron Oxides.

Beneath the harsh deserts of Saudi Arabia lie dark chambers and complex mazes filled with crystalline structures, stalactites and stalagmites.

The limestone floor of the Summan plateau, a karst area to the east of the ad-Dahna desert, is riddled with such caves, known locally as Dahls.

[3] Some have tiny entrances which open into caves, others lead into a maze of passages which can be several kilometers long.