What is something that scientists strongly believed in long ago that today we know is false?

One example is the belief in the phlogiston theory, which posited that a fire-like element called phlogiston was released during combustion. This theory was widely accepted in the 17th and 18th centuries until the late 18th century, when Antoine Lavoisier demonstrated that combustion involves the reaction of a substance with oxygen, leading to the development of modern chemistry. Today, we understand that phlogiston does not exist, and combustion is explained through the principles of oxidation.