Between June 2003 and November 2008, the water level behind the Three Gorges Dam was raised three times from 135m to 156m and then 172.3m. Though the water level had another 2.7m to go before reaching the target 175m, the government felt it had done enough to test out the dam's readiness and called for a halt on Nov 4 2008. The cautiousness stems in part from repeated warnings by environmentalists and scientists that the full impact of billions of cubic metres of water behind the dam is yet unknown. Adding to the complexity of the situation is the changing depth of the water level, which could rise and fall by as much as 30m throughout the year. The water level typically falls during summer when the dam releases excess water to prevent flooding, but rises in winter when the dam is shut. For a region that is already considered geologically and environmentally fragile, what would be the impact from the waters rising and falling by 30m every year? Would it trigger more landslides? Would the repeated soaking and exposing of submerged old cities spread pollution and diseases? An authoritative answer may not available for decades.