In other words, what happens when a terrestrial animal returns to the water permanently? Water is much denser than air, which affects all aspects of an animal's life including movement, sight, and hearing. In addition, animals that return to the water cannot breathe water, and so must return to the surface for air. Despite all these obstacles, many land animals have returned to the water throughout the course of evolutionary history. This lesson will explore many different types of adaptations that modern and extinct animals have evolved to meet these challenges.