This paper studies the rise of a Taylor bubble in a long, vertical liquid-filled conduit such as the riser of an oil production or exploration well. It is shown that, as the bubble rises and expands due to the gradually decreasing hydrostatic pressure, a situation may be reached in which the character of the expansion abruptly transitions from moderate to very violent. The phenomenon can be particularly dangerous with very deep wells, in which a "gas kick," i.e., the intrusion of gaseous hydrocarbons in the riser, can result in a violent and destructive "blowout." As an example, with its expansion, a 1-m-long bubble rising in a 1000-m-long tube causes the liquid to spill out of the top of the tube with a velocity of the order of cm/s for the better part of an hour, until it rises to the order of 10 m/s in the last several seconds. This process is considered to have been responsible for the DeepWater Horizon accident in the Gulf of Mexico in 2010. The same phenomenon underlies volcanic eruptions of the Strombolian type, which are characterized by the intermittent ejection of material up to heights of tens or even hundreds of meters. In this paper the process is studied with a quasiequilibrium model, a dynamic model, and a drift-flux model, all of which give results in substantial agreement with each other. The quasiequilibrium model leads to a very simple relation which, while approximate, gives a robust criterion for the occurrence of violent bubble expansions. The results of an illustrative calculation with OpenFOAM are also presented.