Broaching refers to the release of hydrocarbons at the seafloor after a loss of subsurface well containment. An underground blowout may allow migration of fluids upward through adjacent formations or the annulus to the seafloor. After the BP 001 Macondo well was capped, an oil slick was observed near the well. Available data were interpreted to determine if the slick was reservoir fluid broaching the seafloor or if it was a natural seep in the Gulf of Mexico. The experience revealed that no basis existed for estimating subsurface broach rates or how a potential to broach should be considered during well permitting decisions. Lawrence Berkeley National Laboratory (LBNL) modeled two broaching failure scenarios selected by the Bureau of Ocean Energy Management (BOEM) and representative of Gulf geosystems. Geoscientists from BOEM provided reservoir parameters and interpreted geologic surfaces from depth-migrated three-dimensional seismic volumes. The LBNL adapted existing supercomputing capabilities to model the traveltime for formation fluids to reach the seafloor after subsurface containment failure. The most sensitive parameters affecting traveltime are the mixture of formation fluids and the permeability of the geologic media through which it passes. Assuming nondepletion of reservoir pressure, gas-prone multiphase fluids may broach within a matter of decades. Wet oil and oil-prone multiphase fluids are slower by one to three orders of magnitude. The likelihood for the modeled systems to broach was mainly dependent on casing failure depth. The BOEM concluded that casing failures greater than 7500 ft (>2280 m) below mudline (seafloor) will not broach within a 6-month time frame of regulatory concern.