According to exemplary inventive practice, a deployment line connects a vessel to an anchor, and a tether connects a buoyant electronic unit to the same anchor. The buoyant electronic unit includes a syntactic foam sphere, and a computer and sensors that are housed in the sphere. The anchor and the buoyant electronic unit are discharged from the vessel and sink in the water. The deployment line mechanically detaches from the anchor when the anchor reaches bottom. The buoyant electronic unit stabilizes into an equilibrium position, tethered vertically and tautly to the anchor. Measurements pertaining to phenomena such as underwater electric potential, pressure, magnetic field, and acceleration are taken by the corresponding sensors and are processed by the computer. An electrical (e.g., acoustical) signal is transmitted to detach the tether from the anchor, whereupon the buoyant electronic unit rises to a retrievable position on the surface of the water.