Mushroom diapirs have been attributed to retardation of a rising buoyant source by sinking of denser overburden of similar effective viscosity. New physical and numerical models show that mushroom diapirs also form by reactivation of diapirs that have spread and been buried by overburden with higher effective viscosities. Diapirs were previously thought to become balloon shaped when the viscosity ratio (m value) exceeds unity. However, experimental diapirs reactivated after further burial with an m value of 85 were also mushroom shaped, having peripheral skirts that enfold their former overburden. Denser overburden is entrained because the bulb centers rise faster than their peripheries. The formation of mushroom bulbs by reactivation differs from some previous models that assume all the overburden to be in place from the start.