Why did people believe rivers came from underground, not from rain?

Because for most of history people assumed that rain simply couldn't supply enough water to feed rivers. In ancient times it was widely thought that the land floated on a body of water and that most river water came up from under the earth, a belief already found in the works of Homer around 800 BC. Even the Greek scholars who, by about 500 BC, suspected that much of a river's water came from rain still believed that water rising through the earth contributed a great deal.

The odd part is that pieces of the real picture were spotted early. Around 700 BC the poet Hesiod described vapor drawn up from rivers by windstorms and turning to rain. Hebrew scholars puzzled over the fact that all the rivers run into the sea, yet the sea is not full. And around 350 BC Aristotle correctly wrote that the sun lifts the finest, sweetest water into vapor, which the cold condenses so that it returns to the earth.

Still, the underground theory refused to die. Up to the Renaissance, scholars including Leonardo da Vinci, around 1500, held that rain alone wasn't enough and that water pushed up from the oceans was the main source of rivers. In 1580 Bernard Palissy became the first published thinker to argue that rainfall alone could keep rivers going, which is why he is often credited with the modern theory of the water cycle. Nobody tested it scientifically until 1674, in a study commonly attributed to Pierre Perrault, and the idea only became mainstream science in the early 1800s.

Today we know the ocean sits at the center of the cycle: it holds 97 percent of the planet's water, receives 78 percent of its precipitation and supplies 86 percent of its evaporation. The parts of the cycle also move at wildly different speeds: evaporated water spends about 9 days in the air before falling again, while groundwater can stay underground for more than 10,000 years.