Why does the bottom of the pool appear to be raised?

The bottom of the pool appears to be raised because of the refraction of light when it passes from water to air. Refraction happens because light travels at different speeds in different mediums. 

When light moves from water (denser) to air (less dense), it bends away from the normal line, making objects under water appear closer to the surface than they really are. This bending of light creates an optical illusion that the pool's bottom is raised or shallower. 

This effect is common in swimming pools and other bodies of water, causing the water to look clearer and the bottom to seem closer than it actually is.