In the attack on Dresden, Allied bombers dropped phosphorous bombs to make the streets burn. To get away from the burning, people went into stone ditches which were built in the centers of the boulevards and which were full of water. The original purpose of these ditches had been to put out house fires. The ditches were built at an angle on either side, and 6 feet deep at the bottom. Over time, algae had begun to grow on the sides of the ditches, and therefore in their attempt to flee the bombing, the residents of Dresden slipped into the ditches and drowned because the water was too deep.