By Andrew Cawthorne

HAVANA (Reuters) - Scientific investigators said on Friday they hope to better determine later this year if an unusual rock formation deep off Cuba's coast could be a sunken city from a previously unknown ancient civilization.

"These are extremely peculiar structures ... They have captured all our imagination," Cuban geologist Manuel Iturralde said at a conference after a week on a boat over the site.

"If I had to explain this geologically, I would have a hard time," he told reporters later, saying examination of rock samples due to be collected in a few months should shed further light on the formation off the Guanahacabibes Peninsula on Cuba's western tip.

Iturralde, research director of Cuba's Natural History Museum, has joined Canadian exploration company Advanced Digital Communications (ADC) in efforts to solve the mystery of the smooth, geometrically shaped, granite-like rocks. They are laid out in structures resembling pyramids, roads and other structures at more than 2,000 feet in a 7-3/4 mile-square area.

ADC has suggested they might belong to a civilization that colonized the American continent thousands of years ago, possibly sitting on an island that was sunk to great depths by cataclysmic earth movement such as an earthquake.

That theory, and its inevitable parallel with the myth of the lost city of Atlantis, has provoked skepticism from some scientists around the world who say the depth and age -- ADC has spoken of at least 6,000 years' old -- were not credible.

Some European archaeologists said the stones, stumbled upon in July 2000 while ADC was hunting with sonar equipment for treasure and sunken Spanish galleons, could be formed by natural limestone.

But Iturralde's conclusion that there is no immediately apparent natural explanation for the rocks has lent credence to ADC's theory.

"NEED FOR OPEN MIND"

"It appears like there is some kind of intelligent design in the structure's configuration and planning," ADC's Soviet-born Canadian ocean engineer, Paulina Zelitsky, said on the sidelines of the geophysical conference in Havana.

"I have worked in this field over 30 years and I have never before seen natural structures shaped with such intelligent symmetry and plan. From the very first moment, I was suspecting that these structures were not natural."

While Iturralde gave evidence in his paper on Friday for seismic movement at the site, and possible submerging of the land, he drew short of definitively concluding the rocks were not shaped by nature. If, however, that theory was proven, it would revolutionize understanding of the history of the Americas, he told reporters.

"It would change a lot our knowledge of humans and the evolution of the Americas," Iturralde said.

"Recently, a French archaeologist found some evidence of people being here in South America 40,000 years ago, something we never expect, so you need to be always open to things that you are not expecting, that are not in the framework of present-day knowledge ... We may have found something that nobody has thought about."

ADC plans to take a specially designed robot to the site in a few months to take samples of the rocks and the sediment they are embedded in to try to date them and seek signs they may have once been on dry land. They will also be searching for any sign of human life such as drawings, sculptures or artifacts.

"To drill samples from these structures is not easy because they look like granite. And to drill granite at a depth of 600 meters is very difficult," Zelitsky said.

She said their discoveries could make history. "I think we are talking about the origins of the American continent. There are many hypotheses about how the continent was colonized ... There is quite a controversy, and I think our discovery will be the first physical evidence of the true origins of developed civilization in the Americas."