An optical illusion, caused by atmospheric conditions, in which nonexistent bodies of water are seen:

The answer is; mirage

Mirages are especially common in searing heat when the atmosphere close to the ground is greatly heated by the sun and refracts sunlight hence creating an optical illusion (like a pool of water when in fact it is dry land). The illusion is actually a displaced image of the sky.

stress fractures occur largely in the feet and legs and are common in sports that require repetitive impact, primarily running/jumping sports such as gymnastics or track and field.