'Zero Dark Thirty' Star -- The CRAZIEST Part of Navy SEAL Training Was ...

Chris Pratt went through some real deal Navy SEAL training to prep for his role in "Zero Dark Thirty" -- climbing nets, crawling through sand, jumping over stuff -- but despite all the obstacles the actor tells us there's one part of the program that was particularly brutal.

Pratt -- who also stars on "Parks & Recreation" -- was out in NY yesterday when we talked about the legendary obstacle course at the Naval Amphibious Base Coronado near San Diego.

"I barely touched the tip of what these guys have to go through," Pratt says ... explaining that the REAL training program "helps to boil down the toughest mofos on the planet."

So, what task was the toughest for Pratt? Check out the clip.