By Dr David Whitehouse
BBC News Online science editor
Unique spires tower from the seafloor
Scientists are returning to one of the most remarkable places on our planet: the so-called Lost City of Atlantis.
An expedition from the University of Washington, US, is to use the submersible Alvin to take the first samples from the formation of 18-storey-high hydrothermal vents in the mid-Atlantic.
Formed by superheated water seeping out of the seafloor, the strange structures are made in a different way from other known hydrothermal vents.
They could be home to new forms of life and they could also shed new light on the origin of life on the Earth and on other worlds, too.
The Lost City was discovered by accident in December 2000.
An automated sea-bed explorer stumbled across it near the end of a University of Washington, Scripps Institution of Oceanography and US National Science Foundation expedition to survey the mid-Atlantic.
Alvin will take a closer look
Sketchy details were sent back allowing the submersible Alvin to briefly visit the site before bad weather ended the expedition.
It was dubbed the Lost City of Atlantis soon after, and it became clear that it was like no other hydrothermal vent system yet discovered.
Until then, known hydrothermal vent systems were formed when inky superheated water, saturated with chemicals, seeped from undersea volcanic systems. Towers of deposits formed around the vents that were called "black smokers".
These systems were home to unique ecosystems that lived off sulphur-eating bacteria.
The Lost City appears to be very different from this. It is far older than other vents, with taller towers - up to 55 metres (180 feet) - made of a different material from the dark mottled mix of sulphides in the black smokers.
The system is being heated by old, hot mantle rocks, a million years old - not young volcanism.
Origin of life
The towers are composed of carbonate, the same material as limestone in caves. It appears they were formed by water that is somewhat cooler than the 400 Celsius that forms the black smokers, possibly 40-70 C.
Towers may contain unique life forms
Since it was discovered, the Lost City has been visited by a film crew and Russian scientists, but the forthcoming expedition will be the first time the formation will have been studied in detail.
The researchers will once again use the Alvin submersible for a series of six-hour dives to survey the region and collect samples.
Some believe that the Lost City spires could actually be the most common form of hydrothermal vent on the seafloor, with many others awaiting discovery along the 10,000-kilometre (6,200 miles) Mid-Atlantic Ridge.
The site may have importance for astrobiology as well.
Researchers think that life may have started on our planet in a place just like it, and the same could be happening on other worlds such as under the ice-crusted ocean of Jupiter's moon Europa.
During the expedition, the scientists will be posting a daily report on the internet.