It's an unexpected sight: California's largest lake in the middle of its biggest desert. After the Colorado River flooded in 1905, it took 1500 workers and half a million tons of rock to put the river back on course. With no natural outlet, the artificial Salton Sea is here to stay: its surface is 220ft below sea level and its waters 30% saltier than the Pacific - it's an environmental nightmare that's yet to be cleaned up.