Water levels on the Danube River, the second longest in Europe, have dropped so low because of drought that the hulks of dozens of World War II German battleships — and even the remains of an ancient woolly mammoth — are emerging into view. They are a curious distraction from the summer heat crisis. The Danube's record-low water levels have also forced power plants to the brink of shutting down, and governments across Central and Eastern Europe are taking steps to conserve electricity. In the middle of the river separating Serbia and Romania, near the Serbian port of Prahovo, a...