Internal waves are gravity waves that oscillate within a fluid medium, rather than on its surface. To exist, the fluid must be stratified: the density must decrease continuously or discontinuously with depth/height due to changes, for example, in temperature and/or salinity.
The internal waves travel more slowly than surface waves of similar amplitude because the difference in density between two water masses is much less than it is between air and water. The internal waves typically have wavelengths from hundreds of meters to tens of kilometers and have periods from several minutes to several hours. Internal waves are trapped in the pycnocline of stratified oceans.
It is easier for internal waves to travel between layers of different densities at the pycnocline. In open oceans this is much deeper than the photic zone. This means that those waves travel in complete darkness. This was known recently; however this was portrayed in the Quran 1400 years before it was discovered.
Or like the depths of darkness in a vast deep ocean, overwhelmed with waves topped by waves, topped by clouds: depths of darkness, one above another: if a man stretches out his hand, he will not see it! If Allah does not give light to a person he will not have light!
٤٠ أَوْ كَظُلُمَاتٍ فِي بَحْرٍ لُجِّيٍّ يَغْشَاهُ مَوْجٌ مِنْ فَوْقِهِ مَوْجٌ مِنْ فَوْقِهِ سَحَابٌ ۚ ظُلُمَاتٌ بَعْضُهَا فَوْقَ بَعْضٍ إِذَا أَخْرَجَ يَدَهُ لَمْ يَكَدْ يَرَاهَا ۗ وَمَنْ لَمْ يَجْعَلِ اللَّهُ لَهُ نُورًا فَمَا لَهُ مِنْ نُورٍ
"Waves topped by waves, topped by clouds" are internal waves, surface waves then clouds. In deep oceans you cannot see your hand because there is no light down there. So in deep oceans those internal waves travel in complete darkness.
In the story of Noah it says "waves like mountains":