The Deepwater Horizon is not the only well leaking oil into the Gulf of Mexico for the last month.

A nearby drilling rig, the Ocean Saratoga, has been leaking since at least April 30, according to a federal document.

While the leak is decidedly smaller than the Deepwater Horizon spill, a 10-mile-long slick emanating from the Ocean Saratoga is visible from space in multiple images gathered by Skytruth.org, which monitors environmental problems using satellites.

Federal officials did not immediately respond when asked about the size of the leak, how long it had been flowing, or whether it was possible to plug it.

Skytruth first reported the leak on its website on May 15. Federal officials mentioned it in the May 1 trajectory map for the Deepwater Horizon spill, stating that oil from the Ocean Saratoga spill might also be washing ashore in Louisiana.

The only other mention the Press-Register was able to find of the spill in federal documents occurred in a May 17 transcript of a U.S. Coast Guard media conference. In that transcript, Admiral Mary Landry said that she was unaware there was another drilling rig leaking oil in the Gulf.

Officials with Diamond Offshore, which owns the drilling rig, said that they could not comment on the ongoing spill and referred the Press-Register to well owner Taylor Energy Co., which hired Diamond. Taylor Energy officials did not return calls seeking comment.

Saturday, the Southwings environmental group flew over the Ocean Saratoga with photographer J. Henry Fair of Industrial Scars.com and returned with photos that appear to show a large oil crew boat pumping dispersants into the water at the spill site.

"It appeared the crew boat had barrels of dispersant on board," said Tom Hutchings of Southwings, a volunteer organization of pilots who monitor environmental problems from airplanes.

Henry Fair said that his photos show a large hose coming off the boat and disappearing into the water with several buoys tied to it. It was unclear how far the hose extended underwater.

"I see a hose going over the side. The boat was not moving, but it was making a wake, disturbing the water a lot," Fair said. "I see a glossy slick that one would usually identify as petroleum, and it goes a long way away."

Officials at the National Response Center said that the spill had been reported, but would not say when it began. The U.S. Coast Guard did not immediately respond to e-mails seeking comment.

"We accidentally discovered this spill looking at the Deepwater Horizon images. The question is, what would we see if we were systematically looking at the offshore industry?" said John Amos with Skytruth.org. "Is this an aberration, or are things like this going on all the time? That's why we are calling for public, transparent monitoring everywhere offshore drilling is going on in U.S. waters."

Update (7:06 p.m. June 9):

Company says oil sheen from Ocean Saratoga comes from ongoing work