The Apollo missions included several scientific experiments on the Moon, some of which were dedicated to seismic exploration. The seismic experiments consisted of both passive and active instruments. The passive seismic data were initially analyzed and studied by a group of scientists from Columbia University, the University of Texas, MIT, and the University of Hawaii, and led by Garry Latham. Yosio Nakamura (University of Texas), a member of that group and a leading authority on the internal structure of the Moon and moonquakes, has continued to study those data. Besides measuring natural seismic activities in the Moon's interior, the passive data also recorded man-made impacts such as the S-IVB (third stage of the Saturn launch vehicle) and LM (lunar module) crashing into the surface. The active seismic experiment included the use of three different sources fired either while the astronauts were on the Moon or remotely after they had departed. The active data were studied by a group headed by Robert Kovach (Stanford).