The Prophet (peace be upon him) said:

"No one should sail the sea except the one traveling to perform Hajj, Umrah, or the one who is traveling to fight in the way of Allaah because under the sea lies fire and under the fire lies sea".

This Hadeeth implies a prophecy and a scientific statement; the prophecy has come true and the scientific statement proved to be scientifically true.
Prophet Muhammad (peace be upon him) was illiterate, and lived in an era where no science or scientific technology existed.
Prophet Muhammad (peace be upon him) is talking about the sea, or rather scientific marine secrets that were only unfolded shortly after the WWII; the Prophet (peace be upon him) had never sailed anywhere.

The scientific facet
It was only when WWII exhausted most of the metals in manufacturing weapons that scientists began to search for new reservoirs under water, and there, to their astonishment, they found a long chain of volcanic mountains meandering in the depth of the sea (length estimated 65000 km and depth is 65 km).
The root of such mountain can go deep till it reaches asthenosphere where the temperature is estimated to be 1000oC. Hot current drives tons of molten and burning magma to the seafloor. These also rush upwards in the sea and can sometimes form rocky islands.
So scientific discoveries support the first statement of the Hadeeth, i.e., under the sea there is fire.
It has been noticed that the lava and magma that erupted from the earth's third layer contained water; so there is a layer of earth that is under the sea from which hot lava and magma gush out, and under this fire there is water.

So this scientific finding is also compatible with the second statement in the Hadeeth, i.e., under the fire there is water.
It is noteworthy that all the scientific statements in the hadeeths or the Qur'aan imply prophecies and thus such a fact will be discovered by the scientists one day!