A KING AIR 200 PILOT TAXIED OFF THE TAXIWAY DURING A RAINSTORM RESULTING IN THE AIRCRAFT SLIDING DOWN SLOPE INTO A DITCH. THE AIRCRAFT WAS TOWED OUT OF THE DITCH WITH NO DAMAGE APPARENT.

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I WAS TAXIING ON TO THE RWY AT ZZZ. WHEN I WAS MAKING THE TURN TO THE HOLD LINE; THE L OUTER WHEEL OF THE TANDEM BASE OF THE LNDG GEAR ROLLED OVER THE SOFT SHOULDER OF THE TXWY. THE L OUTER WHEEL DUG INTO THE GND WHICH CAUSED THE WHEEL TO SINK FURTHER INTO THE GND. THE CONDITIONS AT THE ARPT WAS WET; THE WX WAS ABSOLUTELY HORRENDOUS; THE RAIN WAS A CONTINUOUS DOWNPOUR FOR 2 DAYS STRAIGHT. THE WT OF THE ACFT WAS OVER 10000 LBS SO WHEN THE L WHEEL STARTED TO SINK INTO THE SOFT GND THE WT SHIFTED TO THE L AND IT JUST DUG INTO THE MUD EVEN MORE; IT WAS AS IF I WAS IN QUICKSAND. THE L WHEEL STARTED TO SINK AND SIMULTANEOUSLY THE WT OF THE ACFT STARTED TO SHIFT TO THE L. AS THE WT STARTED TO SHIFT; THE WHOLE ACFT STARTED TO SLIDE TO THE L. THE ACFT WAS SLIDING AND HEADING TO THE DITCH. I IMMEDIATELY SHUT THE ENGS DOWN IN ORDER NOT TO DO ANY HARM TO THE ENGS. THE GND WAS MUDDY AND VERY DIFFICULT TO WALK THROUGH SO IT WAS TWICE AS DIFFICULT TO GET THE ACFT OUT. I HAD TO CALL FOR A TOW TRUCK WITH A HIGH CAPACITY TORQUE WINCH TO GENTLY ROLL THE ACFT OUT OF THE MUD. I EVENTUALLY GOT THE ACFT OUT OF THE MUD AND ONTO THE TXWY. A MECH AT THE ARPT INSPECTED THE LNDG GEAR TO MAKE SURE THAT NONE OF THE LINKAGES WERE DAMAGED. IN MY OPINION TO THE CAUSE OF THE INCIDENT WE WERE HURRYING TO TAKE OFF; DUE TO THE FACT WE WERE TRYING TO CATCH A WINDOW OF OPPORTUNITY TO TKOF IMMEDIATELY. A SEVERE TSTM WAS PASSING THROUGH AND WE WERE TRYING TO TAKE OFF SO WE WOULD NOT HAVE TO DEAL WITH THE SYS. IN MY EXUBERANCE TO QUICKLY TAKE OFF I ROLLED OVER THE SOFT PART OF THE SHOULDER AND CAUSED THE INCIDENT.