While Operation Maslenica was in progress, Croatian forces attacked Serb positions to the east. They advanced towards the Peruća Hydroelectric Dam and captured it by January 28, 1993, shortly after Serb militiamen chased away the UN peacekeepers protecting the dam. UN forces had been present at the site since the summer of 1992. They discovered that the Serbs had planted 35 to 37 tons of explosives spread over seven different sites on the dam in a way that prevented the explosives' removal; the charges were left in place. Retreating Serb forces detonated three of explosive charges totaling 5 tons within the high dam in an attempt to cause it to fail and flood the area downstream. The disaster was prevented by Mark Nicholas Gray, a colonel in the British Royal Marines, a lieutenant at the time, who was a UN military observer at the site. He risked being disciplined for acting beyond his authority by lowering the reservoir level, which held of water, before the dam was blown up. His action saved the lives of 20,000 people who would otherwise have drowned or become homeless.