A CL65 DEPARTING YYZ; ON; HAS A RAPID DEPRESSURIZATION DUE TO ACFT DAMAGE. ACFT RETURNED TO LAND AT YYZ.

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ARRIVED AT ACFT WHICH HAD BEEN PARKED ON THE RAMP OVERNIGHT. FO PERFORMED THE EXTERNAL PREFLT WITH NO RPTED DISCREPANCIES. PRIOR TO TAXI; THE RAMP PERSONNEL ASKED US TO MOVE THE ACFT SLIGHTLY UNDER PWR AS THEY COULD NOT REMOVE THE CHOCKS. TAXI TO THE RWY WAS NORMAL (UNEVENTFUL). AFTER TKOF AT GEAR RETRACTION THE AIR NOISE SEEMED LOUDER THAN NORMAL; BUT QUIETED AFTER THE GEAR WAS RETRACTED AND THE ACFT WAS PRESSURIZING NORMALLY. CLBING THROUGH 6000 FT WE EXPERIENCED A LOUD THUD AND A LOT OF AIR NOISE. THE PRESSURE INDICATIONS SHOWED THAT THE ACFT WAS DEPRESSURIZING AT ABOUT 1500 FPM. WE LEVELED AT OUR CLRED ALT AND DECLARED AN EMER RETURNING TO YYZ. UPON INSPECTION OF THE ACFT AT THE GATE; A GASH ABOUT 18 INCHES LONG AND 6 INCHES WIDE AT ITS WIDEST WAS NOTED. THIS WAS LOCATED ABOUT 2-3 INCHES AFT AND SLIGHTLY ABOVE THE R NOSE GEAR DOOR. THERE WAS NO INDICATION THAT WE HIT ANYTHING INFLT AND IT IS POSSIBLE THAT THE ACFT WAS DAMAGED BY GND EQUIP WHILE PARKED OVERNIGHT. IF THIS WAS THE CASE THE DAMAGE WAS MISSED ON THE PREFLT. THE AREA WAS NOT EASY TO SEE UNDER THE ACFT. IT WAS NIGHT. THE L SIDE OF THE ACFT WAS WELL LIGHTED; THE R SIDE IN SHADOW. IF THIS DID HAPPEN; THE DAMAGE CAUSED WAS PROBABLY MUCH LESS NOTICEABLE THAN WHAT WE SAW SUBSEQUENTLY AS THE ACFT DID INITIALLY PRESCRIBE AND I THINK THE DAMAGE WAS EXTENDED INFLT.