A large valve at the top of a well that may be closed if the drilling crew loses control of formation fluids. Casinghead equipment that prevents the uncontrolled flow of oil, gas or mud from the well. By closing this valve (usually operated remotely via hydraulic actuators), the drilling crew usually regains control of the reservoir and procedures can then be initiated to increase the mud density until it is possible to open the BOP and retain pressure control of the formation.