text: Off the main road in this unassuming town, a footpath that snakes through the thick bush and trees of the Nigerian rain forest leads to the remains of what is certainly one of the largest monuments in sub-Saharan Africa: a 100-mile-long wall and moat whose construction began a millennium ago.
  The monument, called Sungbo's Eredo, was erected around a kingdom of the Yoruba -- one of the three main ethnic groups in present-day Nigeria -- and surrounds several towns and villages. Here, the Eredo's earthen bank rises 70 feet in the air from the bottom of a wide ditch, its reddish, vertical wall glistening with patches of moss.
  Few Nigerians have heard of the Eredo; fewer still have visited it. Much of the Eredo lies in ruin, or hidden in the nearly impenetrable rain forest, ignored by locals and Government officials alike.
  Over the last five years, however, a team of Nigerian and British archeologists and preservationists have succeeded in mapping the structure after the work of an earlier archeologist piqued the curiosity of Patrick Darling, an archeologist at Bournemouth University in Britain.
  A carbon analysis of parts of the rampart showed that it dates from the 10th century and suggested that a highly organized kingdom existed in the rain forest at least three centuries earlier than previously believed.
  Because many ethnic groups and cultures have inhabited Nigeria's territory over the centuries and because the country has drawn relatively few archeologists compared with its neighbors here in West Africa, Nigeria is considered a potential treasure trove.