The average depth of the oceans is about 4 km, and oceans cover about 70% of Earth's surface. Make an order-of-magnitude estimate of the volume of water in the oceans. Do not look up any data. (Use your ingenuity to estimate the radius or circumference of Earth. One method is to estimate the distance between two cities and then estimate what fraction of Earth's circumference that distance represents by visualizing the two cities on a globe.)