London, Nov 20 : Reports indicate that Chikyu, the world's largest research drilling vessel, has successfully completed its first mission of finishing the initial 8 week leg of the its research programme in an underwater earthquake zone off Japan's pacific coast.

Chikyu 's long-term mission is to drill up to 6 kilometres below the sea floor to examine conditions deep in the Nankai trough, an earthquake-generating zone off the Pacific coast of Japan.

The first mission of the vessel aimed to drill at the six sites where the vessel will eventually take its cores and build observatories. Measurements were also made to determine the properties of the rocks surrounding each borehole.

But plans were slightly delayed when the bottom 200 metres of the drill, holding the drill bit and the measuring instruments above it, snapped off in one of the holes.

When efforts to fish it out failed, the crew used a spare for the costly piece of. Work was able to continue, and the researchers drilled into four of the six sites as planned, reaching 1,400 metres at the site where they plan to dig 6 kilometres.

"It's a great site and it drilled well," Nature News quoted Harold Tobin, one of the chief scientists on the expedition, as saying.

Chikyu is expected be on site drilling until about 2012. (ANI)