Mountains have roots.
The highest point on Earth is in the Himalayas at Mount Everest 8.8 km high. It turned out that this mountain has roots about 250 km deep.
The larger Asian plate forced the Indian plate deep into the mantle - a process called subduction - sinking it at least 155 miles (250 kilometers) down under the surface, a new study in the May edition of the journal Geology suggests. This plunge is double the depth of previous estimates.
Live Science, Depth of Himalayan Mountain Roots Revealed, 2010
This is correctly portrayed in the Quran by describing mountains as pegs:
[Quran 78.7] And the mountains as pegs
Pegs are used to fix tents... they have some parts above ground and other parts below ground. So the Quran correctly claims that mountains have parts below ground.
How could an illiterate man who lived 1400 years ago have known that mountains have roots?
This subduction occurs in the asthenosphere which turned-out to be semi-molten (melting is the transition from solid state to liquid state).
The asthenosphere is the highly viscous, mechanically weak and ductilely deforming region of the upper mantle of the Earth. It lies below the lithosphere, at depths between approximately 80 and 200 km (50 and 120 miles) below the surface...
The upper part of the asthenosphere is believed to be the zone upon which the great rigid and brittle lithospheric plates of the Earth's crust move about. Due to the temperature and pressure conditions in the asthenosphere, rock becomes ductile, moving at rates of deformation measured in cm/yr over lineal distances eventually measuring thousands of kilometers. In this way, it flows like a convection current, radiating heat outward from the Earth's interior. Above the asthenosphere, at the same rate of deformation, rock behaves elastically and, being brittle, can break, causing faults. The rigid lithosphere is thought to "float" or move about on the slowly flowing asthenosphere, allowing the movement of tectonic plates.
Wikipedia, Asthenosphere, 2019
The rigid lithosphere literally floats above the semi-molten asthenosphere. The melting starts at depths where large mountains have their roots. However this was portrayed in the Quran 1400 years before it was discovered.
[Quran 79.32] And the mountains He anchored.
"Arsaha أَرْسَاهَا" means anchored. Today we know why the Quran used this weird word used for ships floating on water; because the mountains and rigid lithosphere literally float above the semi-molten asthenosphere.
At the outer core the temperature is so high that everything is liquid. The inner core is solid.
How could an illiterate man who lived 1400 years ago have known that mountains float above a liquid?