AN MD80 IN CRUISE FL330 DECLARED AN EMER AND DIVERTED DUE TO LOSS OF CABIN PRESSURIZATION. CAUSE UNKNOWN.

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AT FL330 IN THE VICINITY OF ATL; I WAS EXPLAINING THE MD80'S PRESSURIZATION SYS TO AN INTERN JUMP SEAT PLT. AFTER THE EXPLANATION; MY FO CONTINUED WITH 'AND YOU CAN ALSO CTL IT MANUALLY' AND SWITCHED TO MANUAL CTL. THE CABIN SHOWED A SLIGHT CABIN DSCNT AT 500 FPM. THE FO THEN SWITCHED TO AUTO CTL. THE CABIN SHOWED A CLB OF 500 FPM; FOLLOWED BY A RAPID CLB TO OVER 1500 FPM -- THE CLB NEEDLE WAS PEGGED. THE FO IMMEDIATELY WENT TO MANUAL AND ATTEMPTED TO MANUALLY CLOSE THE OUTFLOW VALVE; RATE OF CLB STILL PEGGED; I ASKED 'ARE WE LOSING THE CABIN?' HE REPLIED 'YES.' AT THAT POINT WE ALL DONNED OXYGEN MASKS; SEATBELT SIGN; AND I STARTED AN EMER DSCNT. FIRST CALL TO ATC WAS NOT RECEIVED BECAUSE I FORGOT TO FLIP A SWITCH TO MASK FROM MIKE. SECOND WAS RECEIVED. I RPTED OUT OF FL330 BECAUSE OF LOSS OF CABIN PRESSURE. I USE TCASII TO AVOID LOWER TFC; AND WE RECEIVED CLRNC TO 15000 FT INITIALLY. FO CONTINUED TO TRY TO RE-ESTABLISH CABIN WHICH HE DID AT AROUND 15000 FT. CABIN OXYGEN MASKS DID DEPLOY. ALL CHKLIST ITEMS REVIEWED COMPLETE. DIVERTED TO ZZZ. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE MAIN FINDINGS ON THE ACTUAL CAUSE OF THE PRESSURIZATION LOSS WERE INCONCLUSIVE. THE RPTR SAID THE MAINT ACTION TAKEN AND THE FAILED COMPONENTS; IF ANY; WERE NOT RPTED. THE RPTR SAID WHILE DISCUSSING THE INCIDENT WITH THE FAA THE IDEA OF SWITCHING FROM A NORMAL OPERATING SYS INFLT TO A BACKUP SYS FOR INSTRUCTION WAS NOT A GOD IDEA.