Researchers created a sonar map of the ancient forest. David Dodd, USM Marine Science. A group of scuba divers recently discovered the remains of an ancient forest about 10 miles off the coast of Mobile, Alabama. Researchers are estimating that the forest existed at least 50,000 years ago, and that it thrived for 1,500-2,000 years before it was buried deep below the ocean floor. The stumps are between six and eight feet in diameter.