A deep magnetotelluric sounding in the French Alps provided a vertical electrical conductivity profile between ~200--1000 km. Two prominent features are observed. First, the conductivity in the depth range 400--800 km is smaller than the conductivity of a pyrolite mantle obtained from laboratory results for a normal geotherm. Second, the data do not require the conductivity to change throughout the transition zone (410--660 km). In this part of the mantle, a temperature of 350--450 C less than normal explains the magnetotelluric conductivity profile. At 200--400 km, our model favors a cold mantle with 1000--1500 ppm of water dissolved in olivine. If correct, this model suggests that the subducted slab is dehydrated before reaching the transition zone.