As you may have heard, before the big BP disaster the government's chief oil drilling regulator let most drilling go forward in the Gulf of Mexico with very little environmental review. Somehow, the Minerals Management Service decided that there was little chance of disaster and thus gave the entire central and western Gulf an exclusion from a requirement for comprehensive environmental reviews.

Yes, you heard that right. Drilling projects in the entire central and western Gulf of Mexico have what the government calls a "categorical exclusion" from detailed environmental studies. The Gulf, by the way, is where most of the nation's offshore drilling takes place. (Here's a handy flow chart from the government showing the approval process.)

How such a broad exclusion was established is an enduring mystery.

On its website, the MMS says categorical exclusions are established "based on experience," and only after "hundreds" of studies have been completed without showing significant impacts.

That raises the question: When did the MMS do so many studies in the Gulf that it decided they were no longer necessary? And who approved that decision and why?

We’ve spent the better part of a month trying to unravel it, and the answer we have so far: The exclusion was created a long time ago, but not even the government knows exactly when or where it came from.

We do know a few things: 1) The White House's Council on Environmental Quality ultimately gives the green light for establishing these exclusions. 2) This particular exclusion likely emerged during the early 1980s -- we know that because Holly Doremus, an environmental law professor at UC Berkeley, appears to have found the earliest reference to this rule in the 1980 Federal Register.

When we called officials at the Council on Environmental Quality, even they explained that they no longer retain the knowledge or the documents relating to why the Gulf exclusion was created, but they did say that categorical exclusions in general first appeared in the late 1970s. CEQ officials sent us to the National Archives for the documents about the Gulf.

The MMS's website does mention the exclusion. That's where we found the language about the exclusion being based on many studies. Here's the full reference:

Some of the MMS categorical exclusions were developed based on experience in reviewing actions for compliance with the National Environmental Policy Act (NEPA) in the past. For example, hundreds of Environmental Assessments (EAs) were prepared for approval of certain types of oil and gas exploration and development and production plans in the Central and Western Gulf of Mexico. However, none of those EAs identified the need to prepare an Environmental Impact Statement (EIS). Therefore, a list of categorical exclusions was established for approval of those plans.

But the agency itself can't seem to explain the origin of the decision. Despite nearly a dozen requests for an explanation with numerous people within the agency, no one responded to our questions.

But if these studies were determined to be no longer necessary three decades ago, consider the state of offshore drilling at the time compared with today:

• Most deep-water wells were around 1,000 feet. (Now, some have surpassed 10,000. The ruptured well in the Gulf is about 5,000.)
• Those wells released only about 1,500 barrels of oil per day. (Now, some wells gush more than 200,000 barrels per day. The Gulf well is spilling 12,000 to 19,000 barrels per day.)

To be fair, it's not as if zero comprehensive environmental studies are being completed. For example, the MMS does region-wide Environmental Impact Studies every five years. Those take about one to two years to complete.

Had a more comprehensive study been done on this individual project, it would have addressed things that would have helped authorities weigh the risks of drilling and prepare for the spill. The plan would have required more details on blowout prevention and worst case spill scenario response.

Another reason why the MMS categorically excludes most drilling projects from these comprehensive Environmental Impact Statements is that the agency is legislatively mandated to approve or deny all drilling applications within 30 days of submission. It's not possible to complete such a thorough review in only a month.

To its credit, the Obama administration has indicated it's looking to change that. President Barack Obama has also expanded the drilling moratorium for all offshore drilling across the U.S.

But efforts before the spill didn't look promising. When the CEQ issued new draft guidance further urging limited use of the categorical exclusion in February, BP pushed back, saying in a letter that categorical exclusions were important for preventing "time delays." Even after the spill, at least 27 new wells were approved using the fast-tracked exclusion process, although officials have said that does not mean new wells have been drilled.