Dawn Watson, 45, found oak trees from a prehistoric forest dating back 10,000 years in the North Sea.
According to experts, the wood could have been hidden since the last Ice Age and was possibly part of a massive forest stretching for hundreds of miles.
"The sea was quite rough by the shore, so I decided to dive slightly further out and after swimming over 300m of sand I found a long blackened ridge," Watson told the Eastern Daily Press.
"When I looked more closely I realised it was wood and when I swam further along I started finding whole tree trunks with branches on top, which looked like they had been felled. It was amazing to find and to think the trees had been lying there completely undiscovered for thousands of years. You certainly don't expect to go out for a quick dive and find a forest."
Watson's partner Rob Spray said: "At one time it would have been a full-blown Tolkien-style forest, stretching for hundreds of miles.
"It would have grown and grown and in those days there would have been no one to fell it, so the forest would have been massive.
"It would have looked like a scene from The Hobbit or The Lord of the Rings, which is something we don't get in this country anymore. Geologists are very excited about it. It was a really miraculous find."
Watson and Spray run the Marine Conversation Society's survey project in East Anglia and hope to find out exactly how long the forest has been there.
The forest was part of a land mass known as Doggerland, which once connected the UK to the rest of Europe.
However, the low-lying land was wiped out about 8,200 years ago by a huge tsunami that created a "North Sea Atlantis".
The Storegga Slide generated huge waves and, coupled with rising sea levels from the time, left Doggerland (which was just 5m above sea level) completely submerged.