Six hundred million years ago the world's waters were teeming with massive, marine monsters, according to Dr. Elizabeth Nicholls, a palaeontologist who lectured at Bermuda Underwater Exploration Institute this week.

The only marine reptile which still exists today is the sea turtle but during the Mesozoic Era - when dinosaurs roamed the land - marine reptiles rivalling the Blue Whale in size were common.

Dr. Nicholls was a key part of a palaeontology team which recently collected the largest fossil remains of one of these “sea monsters” ever found.

The logistically complicated and costly expedition finished bringing the fossil of an Ichthyosaur 23 metres (roughly 69 feet) in length to the Royal Tyrrell Museum of Palaeontology in Alberta, Canada where Dr. Nicholls is employed, in 2000.

Its recovery was made into a documentary for the Discovery Channel.

Previously, the largest marine reptile ever found was 15 metres in length.

The massive creature would have been the largest sea predator in history as well, as Blue Whales, which can reach 30 metres in length, are filter feeders.

The museum is now undertaking the slow process of preparing the fossil for display with an estimated completion date of 2005.

It has been a long road to getting the creature to the museum since Dr. Nicholls first came upon the fossil in 1992.

The team could only work on recovering the fossil, found near a river bed in northern British Columbia, a few months each summer and it took several years to raise the money for the expedition.

The largest commercial helicopter available had to be used to lift out the blocks of fossil cut out of the rock and plastered over two summers.

The skull alone of the large ichthyosaur was almost six metres long.

While these giants of the sea resembled fish in some manner, actually they were formerly land-based animals which have re-evolved and adapted back to the sea, Dr. Nicholls said.

The creatures had lungs rather than gills and had to rise to the surface to breathe. As opposed to fins, their flippers were the evolutionary outcomes of former legs and webbed feet.

These monsters swam the earth's waters for a period of 180 million years when there were no polar ice caps and all the continents were united in a giant land mass known as Pangea.

Part of what was the giant continent's coastline is now the Rocky Mountains and it is along these ridges that some of the most revealing fossils have been found.

Fossils of marine reptiles have also been recovered in China, Germany, Switzerland and the west coast of the United States.

Some of these prehistoric creatures resembled what many conceive Nessie - the Loch Ness monster - to look like.

A long-necked, fat-bodied brontosaurus-like swimmer; Plesiosaurs had the longest necks of any animal known to history with 20 to 25 vertebrae. In contrast, a giraffe has just seven vertebrae.

Dr. Nicholls rejects Nessie theories, however. “All evidence we have has been disproved,” she said when asked about the Scottish legend on Thursday night.

If Nessie was one of these sea reptiles, somehow harboured from extinction, it would have to surface for air regularly, she said, and to be part of a breeding population.

“We would have found him by now,” Dr. Nicholls said.

The oldest sea monsters, the ichthyosaurs like that found in Alberta, had long slim snouts, similar to a swordfish and thick bodies.

The appearance of the sea reptiles varied over their time on the planet, however.

As fossils are recovered and studied, the history of life on the planet is slowly linked together like pieces of a giant puzzle, Dr. Nicholls said.

Even today, we all like to believe in sea monsters like Nessie, she said.

“We all like to believe in sea monsters because the depths of the ocean are like the depths of our subconscious.

“So deep and dark, monsters must be lurking there.”