Under water - day one (Thanks Chris!)

Water is a substance which is 800 times denser than air. As soon as light enters the water, it interacts with the water molecules and suspended particles to cause loss of light, colour changes, diffusion, loss of contrast and other effects. A photo taken under water at one metre distance is not unlike a telephoto above water at 800 metres distance, both looking bluish while lacking contrast. The way light changes under water is responsible for the typical under water 'atmosphere'.