It's speculated that aeons ago this may have been a forest extinguished by the eruption of some volcano now deep under the sea.
It is not believed that there was a forest in the area at all.  
mnli hypothesis: It is not believed that there was a forest in the area at all.   premise: It's speculated that aeons ago this may have been a forest extinguished by the eruption of some volcano now deep under the sea.