"I checked maps, and the data on the maps was white.It said, 'insufficient data to delineate terrain'. Well that got me!" says Dr Roy Mackal, a retired biologist from the University of Chicago.
"It's the end of the world. It gives you a feeling of a surviving prehistoric time.
In the 1980s, Dr Mackal led two expedition teams to the vast Likouala swamp and rainforest area of the Congo which is inhabited by pygmies, on the hunt for this mystery creature - Africa's version of Scotland's Loch Ness Monster.
The Mokele-mbembe is reputed to be a large reptile-like creature, with a long neck, and long tail.
Despite being a herbivore, it is said to roar aggressively if approached by humans. Some say it has a single horn, which it uses to kill elephants.
Many a Western explorer over the years has been gripped by the tantalising possibility that they could discover a creature - a formidable one at that - that has remained, as yet, unknown to science.