Mountains have roots imbedded underground.
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
A peg has some parts above ground and other parts below ground. So the Quran correctly claims that mountains have parts below ground.