PERTH, Australia, April 15 -- The autonomous underwater vehicle Bluefin-21, which was deployed on Monday afternoon for its first underwater search mission, has encountered unexpected depth of waters and ended its mission prematurely, U.S. Navy Captain Mark Matthews told Xinhua on Tuesday.

In an exclusive interview with Xinhua, Matthews said the Bluefin was launched at 5:20 p.m. local time (0920 GMT) Monday evening and was recovered at 1 a.m. (1700 GMT) on Tuesday.

Of the 40 square kilometers of search area initially planned for its first mission, 30 percent had been covered. But no discovery related to the missing Malaysian flight MH370 was made, said Matthews.

"The area where the Bluefin was programmed for searching ranges from a depth of 4,200 to 4,400 meters, according to the chart. How the Bluefin was programmed was to maintain an altitude over the sea floor of 30 meters. Unexpectedly, we encountered an area that was deeper than 4,500 meters and if the Bluefin would have followed its directions, it would have exceeded its maximum operating depth," Matthews said.

The Joint Agency Coordination Center (JACC) said on Tuesday that after completing around six hours of its mission, Bluefin-21 exceeded its operating depth limit of 4,500 meters and its built in safety feature returned it to the surface.

Matthews said the operators made the nautical chart by taking samples of water depth and drew a contour line showing different depth of the bottom. There will be pockets of deeper waters within these samples of water depth.

"It was something the machine didn't expect, the programming didn't tell it how to operate when it hit that kind of condition, so we are adjusting the programming on it and would put it back down," he said.

The initially assigned search area is a 5 kilometers by 8 kilometers rectangle. Matthews said Bluefin will search the area in 7 lines that roughly go north to south. Each of these lines got a sonar system that looks out approximately 350 meters on either side.

Bluefin has completed two lines, which covers about 11.2 square kilometers, or about 30 percent of the planned area, before encountered the excessive depth of waters.

"Data from about 30 percent of the search area has been downloaded and reviewed. There was no sign of any evidence of the aircraft in that portion of the search area," Matthews said.

He said Bluefin has been reprogrammed with better instructions to prevent a similar mission abort and to keep it operating within its operating limitation.

"It will be put back into the water again later Tuesday when the sea state and wave height comes down to allow the safe deployment of the system," he said.

According to Matthews, the re-programming will keep Bluefin at a maximum depth of 4,500 meters as well as following the contour of the bottom. The autonomous underwater vehicle will still keep an altitude of 30 meters above the sea floor until it hits that maximum depth.

He said sonar can reach below 4,500 meters. It is the most effective right now for Bluefin to operate at 30-meter altitude over sea floor, but it's still effective at greater altitude so long as the altitude is not greater than 100 meters.

"You will lose effectiveness if the water gets deeper than 4, 500 meters. However, a majority of the assigned area right now, very high percentage of it, will be within that 4,500 meters, so it should not have an impact on its ability to do a survey search in that area to look for evidence that this area might be the final resting place of MH370."

From the data gathered by Bluefin, searchers had been made aware that the conditions of the sea floor are "relatively barren" with only a few rocks, and "the rocks are not the characteristic of what you see would be an aircraft".

But he stressed that this is normal operating conditions and the operators had worked with that kind of environment before.