The Umm al Samim (also known as the Umm as Samim) is a quicksand area on the eastern edge of the Rub al'khali desert of Saudi Arabia and is within the present-day Omani borders. The waters, such as they are, drain into this brackish low-lying closed basin area off the Omani mountains and the wadis of the Rub al'khali. The Al Samim (known locally as the 'Mother of Poisons' or the 'Mother of Worries') is a salt marsh with a solid-looking crust but can be very treacherous when broken through. There is little vegetation.

Sir Wilfred Thesiger, the traveller and explorer, was the first European to see the area in the late 1940s.