Attempt to explore the secret of the Great Pyramid ends in another mystery

It was a tense moment: the tiny robot had crawled for two hours along an 8ins-wide shaft inside the Great Pyramid of Cheops and drilled a hole in the door at the end. As Egyptologists and television producers held their breath, the live international TV audience saw the camera push through the hole, and reveal another door.

Though the anticlimax might have seemed worthy of Homer Simpson, the researchers insisted they were not disappointed, and the discovery of a door beyond a door was "very important".

Tim Kelly, president of National Geographic's television and film division, said: "I enjoyed the moment of discovery. We were not disappointed. We were successful in our mission." The Pyramid Rover, as it was named, had inched 65 metres up one of two passages stretching from a chamber inside the pyramid, also known as Khufu, to peer through a hole in the door which some thought (or perhaps more accurately, hoped) might hide secret chambers, statues of the pharoah or scrolls dating back 4,500 years.

"We found a space, another sealed chamber," the head of Egypt's Supreme Antiquities Council, Zahi Hawass, said. The tiny camera on the $250,000 robot, built by the Boston firm iRobot, revealed a vertical, sheer stone surface beyond the drilled hole. "That door looks fragile because it has cracks all over," Mr Hawass said. "Maybe something belonging to Khufu is hidden behind the second one. Maybe there is nothing."

The dead end came after a year planning the event, organised by Egypt's Supreme Council of Antiquities, researchers from National Geographic and iRobot.

Archaeologists have been puzzled by the shafts since they were discovered in 1872. Some Egyptologists think the shafts, which rise from an unfinished chamber, were built as vents. Others say they were passages for the dead king's soul to ascend to the afterlife.

The researchers expect to take up to a year of investigation before trying to take any look beyond the second door. It is not yet known whether the US Fox TV network and the National Geographic Channel, which both broadcast the footage live, would again carry the search live.

Initially, it had looked promising as the robot – which uses the same technology as that used to search for survivors at the World Trade Centre – inched towards the limestone door and its two brass handles. In 1993, a German team had managed to send a robot as far as the door, but been unable to get past it.

The Great Pyramid, built 4,500 years ago by Khufu, a ruler also known as Cheops, has four narrow shafts. It is the most magnificent of all Egypt's pyramids, formed by 2.3 million stone blocks, and has lost little of its original height of 481ft (146m) and width of 756ft (230m).

Mr Hawass said the shafts may have had symbolic roles in Khufu's religious philosophy. He proclaimed himself Sun God during his life (pharaohs before him believed they became sun gods only after death) and he may have tried to reflect his ideas in the design of his pyramid. It did not yield the treasures associated with pharaohs, perhaps because it was plundered thousands of years ago.