During the two last decades, the development of γ counting techniques has led to the application of natural radioactivity to the study of numerous processes occurring in the environment. Such work flourished particularly when large HP germanium detectors with low background became available. For some studies, the sample to be measured is small (e.g. sediment, BaSO4 precipitate). In such cases, the use of high efficiency well-type detectors, when compared with similar detectors of conventional shape, provides the best choice. This paper describes three large, well-type detectors used for environmental studies. The background of the crystals has been lowered by: i) placing the detectors in an underground laboratory (Laboratoire Souterrain de Modane, LSM), shielded from cosmic radiation by 1700 m of rocks; ii) using only very low activity materials for the detector assembly and copper-lead shield. Results from various oceanic samples such as particles, sediments, water and hydrothermal sulphides are presented to illustrate the usefulness of this kind of detector for the measurement of environmental radioactivity. For sample volumes of about 10 cm3, these installations are shown to be the most sensitive way to measure nuclides as 226Ra, 228Ra, 228Th and 137Cs.