Dislocation of land caused by gravity.
1400 years ago Arabs knew that sand dunes can cave-in beneath them, however nobody knew that rocky mountains can cave-in as well.
The term landslide or less frequently, landslip, refers to several forms of mass wasting that include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows. Landslides occur in a variety of environments, characterized by either steep or gentle slope gradients, from mountain ranges to coastal cliffs or even underwater, in which case they are called submarine landslides.
Wikipedia, Landslide, 2020
Landslides can occur even in rocky mountains. This was known recently, however this was portrayed in the Quran 1400 years ago.
[Quran 17:26] Are you confident that He will not cause side of land to cave in beneath you? Or unleash a tornado against you? And then you find no protector?
"Side of land to cave in beneath you" today we know it is a landslide that can occur even in rocky mountains.
How could an illiterate man who lived 1400 years ago have known about landslides?