Why must white phosphorus be stored under water?

White phosphorus must be stored under water or another inert liquid because it is a highly reactive, metastable allotrope that spontaneously inflames in air at temperatures as low as 35 °C (95 °F).12

This extreme reactivity is due to the structure of its tetrahedral $P_4$ molecules, which contain strained covalent bonds that are significantly more reactive than those found in other forms of phosphorus, such as red or black phosphorus. Because it reacts vigorously with most reagents and poses a significant fire hazard when exposed to oxygen, submerging it in water provides a necessary protective barrier to prevent ignition.12