Ixtoc 1 was an exploratory oil well being drilled by the semi-submersible drilling rig Sedco 135 in the Bay of Campeche of the Gulf of Mexico, about northwest of Ciudad del Carmen, Campeche in waters deep. On 3 June 1979, the well suffered a blowout resulting in one of the largest oil spills in history. Accident Mexico's state-owned oil company Pemex (Petróleos Mexicanos) was drilling a deep oil well when the drilling rig Sedco 135 lost drilling mud circulation. In modern rotary drilling, mud is circulated down the drillpipe and back up the wellbore to the surface. The goal is to equalize the pressure through the shaft and to monitor the returning mud for gas. Without the counter-pressure provided by the circulating mud, the pressure in the formation allowed oil to fill the well column, blowing out the well. The oil caught fire, and Sedco 135 was extensively burned then scuttled. At the time of the accident, Sedco 135 was drilling at a depth of about below the seafloor. The day before Ixtoc suffered the blowout and resulting fire that caused her to sink, the drill bit hit a region of soft strata. Subsequently, the circulation of drilling mud was lost resulting in a loss of hydrostatic pressure. Rather than returning to the surface, the drilling mud was escaping into fractures that had formed in the rock at the bottom of the hole. Pemex officials decided to remove the bit, run the drill pipe back into the hole and pump materials down this open-ended drill pipe to seal off the fractures that were causing the loss of circulation. During the removal of the pipe on Sedco 135, the drilling mud suddenly began to flow up towards the surface; by removing the drillstring the well was swabbed (an effect observed when mud must flow down the annulus to replace displaced drill pipe volume below the bit) leading to a kick. Normally, this flow can be stopped by activating shear rams contained in the blowout preventer (BOP). These rams are designed to sever and seal off the well on the ocean floor; however, in this case, the drill collars had been brought in line with the BOP and the BOP rams were not able to sever the thick steel walls of the drill collars leading to a catastrophic blowout. The drilling mud was followed by a large quantity of oil and gas