The Fenno-Skan high voltage direct current (HVDC) power line links Finland and Sweden across the Bothnian Sea. A 180-km long cable has been installed at the sea bottom. A return current up to 1280 A flows through water and sea-bottom rocks. This source gives a unique possibility for large-scale DC sounding. First test measurements were done by the Department of Geophysics, University of Oulu in May, 1991. In June, 1992 a bilateral experiment was carried out by Finnish and Russian geophysicists. On the base of 42 sites measured during two field seasons apparent resistivity values were calculated. For the interpretation of the measured data a thin-sheet approximation was used. The model consisted of two thin sheets. The uppermost sheet had a conductance S due to the sea water and sediments according to a priori information, while the second one had a depth-integrated resistivity R due to the Earth's upper crust. A good conductor was located beneath. Comparisons of the experimental and theoretical apparent resistivities show that the R value for the Earth's upper crust in southwestern Finland is about 3 × 10 8 Ωm 2, which is in good agreement with the value calculated from the magnetotelluric (MT) data in the Proterozoic Central Finland Granitoid Complex. However, it is more than a factor of 10 lower than the value calculated similarly from the MT data in the Kuhmo region in the Archaean part of the Fennoscandian Shield in eastern Finland and the reported value based on the magnetohydrodynamic (MHD) data in the Kola Peninsula. The results do not reveal any distinct crustal conductors within the research area. This experimental indicates that deep geoelectrical resistivity studies can be carried out successfully at rather large distances, at least 50 km, in the surroundings of a high voltage, DC power link.