After an uneventful landing, a Boeing 737-400 aircraft operated by Tailwind, registration TCTLE, was taxiing from runway 34 to taxiway E3 at Zurich Airport. The runways and taxiways were wet and nighttime conditions prevailed. A few metres after the aircraft had passed the turn-off for de-icing lane F2 on taxiway F, a mound of earth to the south of the commander made him feel uncertain and he turned to the right in order to cross the de-icing pad (DIP) F and join taxiway F south of it. After this 55 degree turn to the right the aircraft left the hard surface of taxiway F3 and came to a standstill on the grass triangle between de-icing lanes F2 and F3 and de-icing pad F.