Gushes hot water from deep underground.
Geysers are one type of hydrothermal vents. They gush hot water from deep underground.
Oceanographers found hydrothermal vents at the bottom of the ocean gushing hot water at more than 400 ºC.
A vent site in the Cayman Trough named Beebe, which is the world's deepest known hydrothermal site at ~5,000 m (16,000 ft) below sea level, has shown sustained supercritical venting at 401 ºC (754 ºF) and 2.3 wt% NaCl.
Wikipedia, Hydrothermal Vent, 2018
1400 years ago people could only have seen geysers at the surface but nobody could have seen this at the bottom of the ocean.
[Quran 81.6] And if the oceans were heated.
Sujirat "سُجِّرَتْ " in Arabic means heated. Well is 400 ºC hot enough?
How could an illiterate man who lived 1400 years ago have known that oceans are being heated?