IXTOC 1 Oil Well Blowout, Bay of Campeche, Gulf of Mexico, June 1979 to March 1980. Well head aerial view from NOAA.http://www.cedre.fr/en/spill/ixtoc/ixtoc.php
NOAA Estimate: 140,000,000 gallons of oil spilled from Campeche eruption
Cedre: "On 3 June 1979, in the Gulf of Mexico (Bay of Campeche), some 80 km from Carmen town, Ixtoc 1's offshore drilling rig, set up by the Perforaciones Marinas del Golfo on behalf of the national company Petroleos Mexicanos (Pemex), was destroyed by the blast of an oil eruption. A fire broke out. This type of accident is quite rare, although it is not one of a kind.
...This eruption lasted far longer. It was only stopped on 23 March 1980, after 295 days, during the which the oil spurt had been reduced first from 4,200 - 4,300 tonnes/day to 1,400 - 1,500 tonnes/day, thanks to the digging of freeing pipes which lowered pressure in the implicated well.
"The total quantity of oil spilled at sea will never be known exactly. The more cautious estimations suggest some 470,000 tonnes were spilled, while in the worst case scenario it could be as much as 1,500,000 tonnes. Between half and a third of this oil burned, causing a vast atmospheric pollution. The remaining part spread over the Gulf of Mexico in the form of drifting slicks. ..."
Cedre (France): Centre of Documentation, Research, and Experimentation on Accidental Water Pollution
NOAA Major Oil Spills: Ixtoc 1
"The Ixtoc I exploratory well blew out on June 3, 1979 in the Bay of Campeche off Ciudad del Carmen, Mexico. By the time the well was brought under control in 1980, an estimated 140 million gallons of oil had spilled into the bay. The Ixtoc I is currently #2 on the all-time list of largest oil spills of all-time, eclipsed only by the deliberate release of oil, from many different sources, during the 1991 Gulf War...."
NOAA: Oil Spill Case Histories: 1967-1991 (PDF)
IXTOC Oil Spill Assessment: Final Report Executive Summary (PDF via Smithsonian)
Prepared for US Bureau of Land Management
NOAA deepwater Horizon Incident Page
NOAA Photos of OIl Spills
Wikipedia: Oil Spills