Why does the bottom of a tank or a pond containing water appear to be raised?

The bottom of a tank or a pond containing water appear to be raised. This occurs due to the 'Refraction of Light'.

The light from the bottom of the tank passes from water to air to reach the observer. Water is a denser medium and air is a rarer medium. So the light rays undergo refraction and bend away from the normal.

Because of the bending of light rays, they appear to be coming from smaller depths.