Rocks sink underground.
1400 years ago people thought that rocks can fall to the ground and that's it. Nobody believed that rocks sink further underground. However today we know that this is false. Rocks can sink hundreds of kilometers deep.
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 subduction carries water inside rocks from the surface down to the middle mantle. Scientists found huge amounts of water inside a mineral 410 to 660 km below the surface.
The deep Earth holds about the same amount of water as our oceans. That's the conclusion from experiments on rocks typical of those in the mantle transition zone, a global buffer layer 410 to 660 kilometers beneath us that separates the upper from the lower mantle.
"If our estimation is correct, it means there's a large amount of water in the deep Earth," says Hongzhan Fei at the University of Bayreuth in Germany. "The total amount of water in the deep Earth is nearly the same as the mass of all the world's ocean water."
The results add to mounting evidence that there is much more water than expected beneath us, mostly locked up within the crystals of minerals as ions rather than liquid water.
New Scientist, There's as much water in Earth's mantle as in all the oceans, 2017
The high pressure down there breaksdown all rocks. But from this discovery we know that the water and rocks from the surface sank more than 400 km beneath Earth's surface. However this was portrayed in the Quran 1400 years before it was discovered.
[Quran 2:74] Then after that your hearts hardened like rocks, or even harder. For there are some rocks from which rivers gush out, and others that crack and water comes out from them, and others that descend in awe of Allah. Allah is not unaware of what you do.
"Others that descend in awe of Allah" here the rocks and water are descending but the Quran didn't say "rocks" or "water" anymore because at that depth rocks break down and the water is not liquid anymore but rather in the form of ions inside a mineral. No mistakes in the Quran.
How could an illiterate man who lived 1400 years ago have known that rocks can sink inside Earth?
For "there are some rocks from which rivers gush out" see Porous Rocks.
For "others that crack and water comes out from them" see Rocks Crack.