The study of the electrical conductivity structure of the lower crust and the upper mantle and midmantle promises to place important constraints on the thermobaric and compositional state of the earth's interior. Such work, particularly for depths above approximately 400 km, is best attempted by magnetotelluric sounding, an effort frustrated by the almost complete absence of stable long-period electric field measurements. In this paper, we propose that proven techniques developed for seafloor magnetotellurics be applied also to deep freshwater lakes. The establishment of such long-period electric and magnetic observatories in the thermally stable, low-noise lake bottom environment will yield new in sights on the deep electrical structure beneath the continents. A lake bottom long-span electric field array was deployed in a deep freshwater lake adjacent to Seattle, Washington. A vector flux gate magnetometer was buried on the shore of the lake, and magnetotelluric data of high quality were collected. The analysis of the data described in this work clearly demonstrates the feasibility of establishing such lake bottom continental magnetotelluric observatories. Inversion of the data provides a view of structure beneath the site consistent with regional seismic and tectonic models. ¿American Geophysical Union 1987