B767-300 DEVELOPED A 'BODY DUCT LEAK' DURING CRUISE. SMOKE WAS REPORTED BY THE FA; SO THE FLT CREW DECLARED AN EMER AND LANDED AT THE NEAREST ARPT.

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PASSING OVER ZZZ AT FL240; WE HEARD A THUMP IMMEDIATELY FOLLOWED BY A SIGNIFICANT PRESSURE CHANGE IN OUR EARS. I IMMEDIATELY LOOKED UP TO THE TINY CABIN PRESSURE GAUGES TO SEE IF WE WERE LOSING CABIN PRESSURE; IN ANTICIPATION OF A RAPID DEPRESSURIZATION. I COULD TELL THAT MY JUMPSEAT RIDER WAS THINKING THE SAME THING AS WE BOTH SIMULTANEOUSLY LEANED OVER THE CTR CONSOLE AND LOOKED UP AT THE GAUGES. IT WAS APPARENT THAT WE WERE NOT LOSING PRESSURE; AND QUERIED EACH OTHER AS TO WHAT WE THOUGHT THE NOISE AND PRESSURE JOLT MIGHT BE. AT THAT TIME WE GOT A BODY DUCT LEAK EICAS MESSAGE. THE FO WAS FLYING AND I OPENED THE QRH TO THE BODY DUCT LEAK CHKLIST. DURING THIS TIME; THE CABIN CALL CHIME WAS RINGING; BUT WE WERE NOT ABLE TO HEAR ANYTHING OVER THE INTERPHONE. WITHIN SECONDS; THERE WAS RAPID AND LOUD POUNDING ON THE COCKPIT DOOR. MY JUMPSEATER ASKED PERMISSION TO OPEN THE DOOR; WHICH I GRANTED. THE FLT ATTENDANT SAID THERE WAS A LOT OF SMOKE AND DEBRIS IN THE AFT CABIN; AND IT LOOKED LIKE THERE WAS A FIRE. THE FLT CREW IMMEDIATELY PERFORMED THE INITIAL ACTION ITEMS FOR SMOKE; AND PUT OUR FULL-FACE OXYGEN MASKS ON AND ESTABLISHED COM. I LOOKED AT MY HSI AND SAW THAT WE WERE JUST A LITTLE SW (MAYBE 20-30 MI) FROM ZZZ. I INFORMED THE CREW THAT WE WERE GOING TO DIVERT TO ZZZ; CALLED ATC; DECLARED AN EMER FOR SMOKE IN THE CABIN; REQUESTED AN IMMEDIATE DSCNT AND A TURN DIRECT. WE WERE IMMEDIATELY CLRED FOR A R TURN DIRECT TO ZZZ WITH A DSCNT TO 11000 FT. THE 'BODY DUCT LEAK' CHKLIST WAS QUICKLY COMPLETED AS I THOUGHT THIS MIGHT HELP THE SITUATION IN THE BACK OF THE PLANE; AND THE COPLT AGGRESSIVELY TURNED THE PLANE TOWARDS ZZZ AND DSNDED WITH FULL SPD BRAKES. GIVEN THE INFO I HAD RECEIVED SO FAR; I BELIEVED THE SAFEST COURSE OF ACTION WAS TO TREAT THIS EMER AS AN INFLT FIRE. THERE HAS BEEN AN EMPHASIS ON THE SMOKE AND FIRE SCENARIO OVER THE YRS IN THE COMPANY'S INITIAL AND RECURRENT TRAINING PROGRAMS. ESTABLISHING AND MAINTAINING COM WITH THE BACK OF THE ACFT IS CRITICAL. WITH THE COPLT FLYING; I ATTEMPTED TO TALK WITH THE CREW IN THE BACK OF THE PLANE. THE STATIC WAS OVERWHELMING AND MADE THE USE OF THE INTERPHONE NEARLY IMPOSSIBLE. I TURNED TO MY JUMPSEATER AND ASKED HIM TO WORK THE INTERPHONE; PA'S; AND GET ME WHATEVER INFO HE COULD; CONCERNING OUR SITUATION. BTWN THIS TIME AND LNDG; I NOTED MENTION OF SMOKE AND ALSO A LESSENING OF SMOKE. IT APPEARED THINGS WERE NOT GETTING WORSE. I TURNED MY ATTN TO GETTING THE ACFT ON THE GND. WE RECEIVED VECTORS TO AN ILS. ALL NORMAL CHKLISTS WERE ACCOMPLISHED. (NOTE: THE APCH PLATES FOR ZZZ WERE IN OUR NEW AIRWAY MANUAL; AND THIS WORKED WELL.) THE FLT CREW PERFORMED THEIR DUTIES WEARING FULL-FACE OXYGEN MASKS. THE WX WAS BETTER THAN 2000 FT AND 5 MI WITH WINDS NEARLY DOWN THE RWY AT ABOUT 12-15 MPH. I CHOSE TO HAVE MY COPLT FLY THE APCH AND LNDG; SO THAT I COULD CONTINUE TO MONITOR THE SITUATION IN THE BACK; AS WELL AS CONTINUE BRIEFING WHAT I PLANNED TO DO AFTER LNDG. THERE WAS SO LITTLE TIME; AND I WANTED TO BE SURE EVERYONE KNEW WHAT TO EXPECT; AND WHAT WAS EXPECTED OF THEM. MY BIGGEST CONCERN AT THIS TIME WAS WHETHER WE WOULD NEED TO EMER EVAC ON THE GND; OR THAT PANICKED PAX WOULD OPEN DOORS. THE JUMPSEATER ASSURED ME THAT THE PAX HAD BEEN INSTRUCTED TO STAY IN THEIR SEATS UNTIL THEY RECEIVED INSTRUCTION FROM THE CREW. THE REST OF THE PLAN WAS FOR HIM TO LEAVE HIS SEAT AS SOON AS THE ACFT WAS STOPPED; QUICKLY ASCERTAIN THE SITUATION AND ESSENTIALLY BE MY EYES AND EARS AGAIN. I HAD NO INTERPHONE FOR COM WITH THE CABIN. FO'S LNDG WAS EXCELLENT. AFTER STOPPING ON THE RWY; THE JUMPSEATER PROVIDED ME WITH THE INFO I NEEDED TO DECIDE AN EMER EVAC WOULD BE UNWARRANTED. THE PAX WERE AGAIN TOLD TO REMAIN SEATED; AND THAT AS A PRECAUTION THE ARPT FIRE CREW WOULD LOOK OVER OUR ACFT PRIOR TO TAXIING CLR OF THE RWY. THE FIRE CREW EXAMINED THE ACFT AND INFORMED US THERE WAS NO SIGN OF SMOKE; FIRE; OR DAMAGE. WE THEN CLRED THE RWY AND REQUESTED A GATE. THIS TOOK SOME MINS; AS THERE WAS SOME UNCERTAINTY ASTO WHETHER A B767 WOULD FIT AT ANY OF THE GATES. I INFORMED THE CREW THAT WE WOULD DEPLANE NORMALLY THROUGH THE JETWAY.