As when Allah (swt) says:

{ وَإِذَا الْبِحَارُ سُجِّرَتْ }

(And when the seas are filled with flame) (Al-Takwir: 6)

These are futuristic signs that will happen on Judgment day; the seas will erupt and become one sea where the fresh will mix with the salty. Sea in language is the place where the water accumulates whether it is fresh or salty. Hence, eruption of seas means releasing them on the earth’s surface.