The Water Canopy Theory is a hypothesis  primarily found within young Earth creationism,  

which posits that before the Biblical flood,  Earth was enveloped by a "canopy" of water vapor.

Proponents of this theory suggest that this  canopy created a greenhouse-like effect,  

leading to a much milder climate globally  and potentially contributing to the  

longer lifespans and lush vegetation  described in early Biblical texts.

The theory proposes that this  water canopy eventually collapsed,  

contributing significantly to the great  flood described in the story of Noah's Ark.

Critically, this theory is not supported by  mainstream science. Scientists argue that  

a canopy of water vapor of the magnitude  suggested would not be physically stable  

and would create a heat retention scenario  that could render the planet uninhabitable.

Moreover, geological and atmospheric evidence  does not support the existence of such a canopy.