BEIJING, June 14 -- Ten divers began a seven-day search for a possible underwater "Atlantis" on Friday in the Fuxian Lake near Kunming, the second-deepest freshwater pool in the country.

    Local diver Geng Wei first told of a large ancient city in the lake eight years ago, thought to span 2.4 square kilometres. Geng claimed to have seen lots of square boulders more than 1.4 square metres in size, either piled or scattered deep underwater.

    In 2001, the local government launched the first large exploration of the lake, which was broadcast live across the nation by China Central Television (CCTV).

    A submarine was sent down and detected a 60-metre-long stone wall. Divers unearthed a shard of pottery embedded in the stone wall, which was found to date back to the Han Dynasty (104 BC-220 AD).

    The evidence convinced Chinese archaeologists that there might be some constructions under the lake, possibly more than 1,800 years old.

    This hypothesis was substantiated on Friday in the first dive, when Geng was videotaped finding three notches, each 1.2 metres long and 45 centimetres wide, on a moss-covered square slate.

    The "IY"-shaped notches must have been artificial, and "support the idea that all the stones were once processed by humans," said Li Kunsheng, director of the Archaeology Research Centre of Yunnan University.

    But Liu Qingzhu, director of the Institute of Archaeology under the Chinese Academy of Social Sciences, added: "We still have not enough information to verify that these slates made up a city. Even the shard and shell cannot represent the exact date of the rocks."

    After Geng announced his discovery eight years ago, more claims were made of underwater finds in the lake, which boasts a water surface of 212 square kilometres and an average depth of 87 metres. They include a slate path, an arena-like building and a small pyramid.

    However, Liu, who was present during two underwater excavations, said no pictures or evidence about the above "findings" had ever been provided by these people.

    Despite this, experts have engaged in a prolonged debate over whether these slates are relics of a documented city that mysteriously disappeared.

    The history books show that the city of Yuyuan to the north of the Fuxian Lake once existed, but it disappeared from records after the Southern and Northern Dynasties (AD 420-589).

    Li said the lake is situated on an earthquake-intense belt, which might suggest that the underwater construction may have sunken in rising waves during a quake.

    Dissenters argue that the stone structure is contrary to buildings of this era, which were made of bamboo, wood or mud.

    Liu said that while all the answers to this underwater mystery will not be found in seven days, "we'll try to outline a layout map of what is beneath, and do more in the future."