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Ndamukong Suh is so serious about a construction career after the NFL, he took an UNPAID INTERNSHIP to learn the ropes.

Then again, when you're making $114 MILLION ... do you really need the extra cash?

It's actually a pretty cool story ... Suh locked up a 3-day internship working for Related Companies -- a multi-billion dollar company founded by Dolphins owner Stephen M. Ross.

Okay, so maybe Suh got the hookup 'cause he's tight with the boss man -- but sources connected to the internship tell us Suh is very serious about construction (it was his major at Nebraska).

We're told Suh visited several construction sites -- including a new $15 BILLION project in lower Manhattan -- and has been allowed to attend important building meetings.