DURING TAKEOFF, THE PLT RADIOED THAT A DOOR HAD POPPED OPEN & HE NEEDED TO RETURN. ACCORDING TO A PLT WITNESS, THE ACFT HAD CLIMBED ABOVE TREETOP LEVEL WHEN IT STARTED TO LOSE ALT. ANOTHER WITNESS SAID THE ACFT LEVELED OFF & STARTED TO SINK, THEN THE NOSE CAME UP AGAIN, BUT THE ACFT SETTLED BACK TO THE GROUND & ROLLED OFF AN EMBANKMENT. AFTER GOING OVER THE EMBANKMENT, THE ACFT BECAME AIRBORNE AGAIN AND IMPACTED IN A NOSE DOWN ATTITUDE. THE GROUND WITNESS RECALLED THAT WHEN THE ACFT PASSED HIS POSITION NEAR THE RWY, THE DOOR WAS OPEN & AN OBJECT THAT LOOKED LIKE A SEAT BELT WAS HANGING OUT THE TOP OF THE DOOR.AN EXAM OF THE DOOR REVEALED NO PREIMPACT PROBLEM THAT WOULD HAVE PREVENTED IT FROM CLOSING PROPERLY. IMPACT MARKS WERE FOUND ON THE GROUND NEAR THE END OF THE RWY WHERE THE LANDING GEAR HAD TOUCHED DOWN HARD, BEFORE THE ACFT WENT OVER THE EMBANKMENT. ACCORDING TO THE OPERATOR'S MANUAL, AN UNLOCKED DOOR WOULD NOT AFFECT THE FLT CHARACTERISTICS, EXCEPT TO SLIGHTLY REDUCE THE CLIMB RATE.