On April 20, 2010, a mayday call from the Deepwater Horizon drilling rig in the Gulf of Mexico reporting an explosion aboard signaled the beginning of one of the largest man-made environmental disasters in the United States. Today, the environmental, economic, and political repercussions of the oil spill are just being felt - but what happened the day the rig went ablaze, before the magnitude of the catastrophe as we know it today was clear?

National Geographic Channel obtained exclusive never-before-seen footage from the U.S. Coast Guard, NGC production crew and salvage company who responded in the chaotic first 36 hours following the explosion. From NGC's documentary Gulf Oil Spill:

This unparalleled documentary of the first 36 hours includes coverage of the rig engulfed in flames, local vessels dousing the burning rig with water to stem the blaze and intense, close-up moments of the burning vessel sinking into the abyss. Coast Guard members speak emotionally about searching the seas for missing crew, and teams of mariners and engineers discuss trying to fight the fire at sea and save the vessel, ultimately to no avail.

“I could see the glow of the burning rig at 90 miles away,” says USCG Lieutenant Lim. “I knew this was big … like seeing hell on earth.” Added Kurt Peterson, an elite swimming rescue member who worked to save the rig’s drowning and injured crew, “I never had to deal with so many people at one time … since Hurricane Katrina.”

Salvage team masters describe their experience after working desperately to save the vessel and then watching it sink: “When you lose a vessel, it’s a somber moment. You feel the loss, you feel the failure … unfortunately for us, we were fighting a losing battle from the start. Unless that oil was cut off, there wasn’t really too much anybody could have done to prevent the eventual outcome that we witnessed.”

Finally, NGC interviews 23-year-old survivor Chris Choy, who says, “I kept thinking — we’re all dead … I didn’t know if I was the only person still on the rig or what.” He continues, “It’s something I’ll never forget, images just burned into my head .… We got together in a big group and we figured we were missing 11 guys .… We just kept praying that someone would find them.”

Here are some gruesome yet fascinating photos of the explosion that sunk Deepwater Horizon oil rig:

The off-shore oil rig Deepwater Horizon in flames off the coast of Louisiana. An estimated 126 people were aboard the Deepwater Horizon at the time of the explosion.

More info: National Geographic Channel Gulf Oil Spill Website | Program airs Thursday May 27, 10 PM ET/PT

(All images ©Steadfast TV, courtesy of National Geographic Channel)