Rainforests are considered the "lungs of the Earth" because they play a crucial role in producing oxygen and absorbing carbon dioxide through the process of photosynthesis. The vast amount of trees and plants in the rainforest continuously release oxygen into the atmosphere, which is essential for the survival of all living organisms on Earth. Additionally, they help regulate the global climate by absorbing carbon dioxide, a greenhouse gas that contributes to global warming. This vital role in balancing oxygen and carbon dioxide levels makes rainforests essential for maintaining the health of our planet.