Why must white phosphorus be stored under water or another inert liquid?

White phosphorus must be stored under water or another inert liquid because it is a highly reactive and metastable substance. Due to the strain within its tetrahedral P₄ molecules, it is far more chemically reactive than other allotropes of phosphorus. Specifically, it inflames spontaneously in air at temperatures as low as 35 °C (95 °F). Storing it under an inert liquid prevents contact with oxygen, thereby mitigating the risk of spontaneous combustion.1