British scientists have embarked on a mission to study huge spots on the Atlantic seabed where the earth's crust is missing - an enigma that defies geophysical theory and provides an unprecedented peek at the planet's green interior. The 12-member expedition left the Canary Islands Monday with a new high-tech vessel and a robotic device named Toby that will dig up rock samples at the site and film what it sees. The main spot - there is at least one other in roughly the same area and a third is suspected - is about 3,500 meters under the surface of the Atlantic and located about 2,000 nautical miles southwest of the Spanish archipelago off Africa's coast. It is part of a globe-spanning ridge of undersea volcanos, the kind of structure that forms when Atlantic tectonic plates separate and lava surges upward to fill the gap in the earth's crust. But that did not happen this time. Where there should be a 7-kilometer-thick layer of crust, there is instead that much mantle - the very dense, leafy-green rock that makes up the center of the Earth. Scientists have seen chunks of mantle that spewed up with lava, but never a large, exposed stretch of seabed like this one. "It is like a window into the interior of the earth," Bramley Murton, a geophysicist who is taking part in the six-week mission, said Tuesday from the research ship RRS James Cook as it headed to the site, still five days away. This spot is irregularly shaped, about 50 kilometers long and perhaps that distance or more at its widest. It was detected about five years ago with sonar from a surface vessel. There are two main theories as to what happened, Murton said: a fault ripped away huge chunks of crust, or in an area of crust-forming volcanoes this spot was mysteriously left out, Murton said. Fellow geophysicist Roger Searle of Durham University, one of the lead researchers, said the study aims to provide insight on everything from the chemistry of oceans to the mechanisms of how the Earth behaves under so much water. The robotic device will land on the exposed mantle, deploy a drill and dig into the mantle to bring back samples. The project is being financed by Britain's National Environment Research Council and the Department of trade and Industry's Large Scientific Facilities Fund.