B.o.B Diss Track In Ya Mouth, Neil deGrasse Tyson ... EARTH IS FLAT!!

B.o.B just made rap history -- dropping the first ever diss track targeting a famous astrophysicist, while also declaring the Earth is flat.

You might have heard about B.o.B's beef with Neil deGrasse Tyson -- the science genius and sometimes TV host -- over the shape of our planet. It was already funny ... B.o.B tweeted about the horizon and such, and Neil tried to put him in check with facts and stuff.

But B.o.B took it to the next level with this track, "Flatline." You gotta listen -- direct shots at Neil, NASA and, for good measure, the Masons.

Just when you thought Neil dropped the mic by calling B.o.B 500 years behind the times ...

Actually, he kinda did. See what happens when trolling goes unchecked?