What was found
(CAN) WHILE ENROUTE, ON A POLAR ROUTE IN THE MIDDLE OF HUDSON BAY, THE FLIGHT CREW REPORTED ACRID SMOKE COMING OUT OF OVERHEAD PANEL IN THE AREA OF THE TERRAIN SWITCH AND THE FAILURE OF THE GPWS. THE SMOKE THEN DISSIPATED. THE CREW DECLARED A PAN PAN (URGENCY), AND RETURNED TO YYZ, WHERE THE FLIGHT LANDED 30,000 KG OVERWEIGHT. (TC NR 20060725001)