Two large unannounced events occurred at sea in aseismic areas in the Atlantic. Comparison of these with the announced event Chase III shows them to be explosions.Large explosions at sea may be recognized by the relatively small amplitude of long period surface waves with periods up to 10 s. Energy of longer periods is absent for events mb ≤ 5.5. The surface wave magnitudes for the events are at least 1.5 smaller at 10 s than those of underground explosions of equal mb, at 20 s they are at least 0.9 smaller. At longer periods the difference between body wave and surface wave magnitude is larger than 0.9 but larger explosions are needed to determine the separation. Underwater explosions on or near the continental shelf are very efficient in the generation of higher mode short period waves.