A hydroelectric dam under construction in southeast Laos collapsed late Monday, July 23rd  in the Attapeu province of Sanamxay district, releasing an estimated half-billion cubic meters of water.  The collapse has left hundreds missing and an unknown number dead as water went gushing into six villages.  Approximately 6,600 people have been left homeless as a result of the flood.   The dam, which was under construction and not yet in operation collapsed after heavy monsoon rains in the area.