Coal Galore Edition!
>“We think there are between three trillion and 23 trillion tonnes of coal buried under the North Sea,” explained Dermot Roddy, former professor of energy at Newcastle University.

This is enough coal to power the world for 500 years to 3000+ years at current rates of consumption. And it's in one coal field. Many more are out there.
Also, mining under the sea has been done in that region since at least the Medieval Period.
https://www.worldcoal.com/coal/31032014/coal_discovered_in_north_sea_674/