An account is given of the seismic work carried out on the world cruise of H.M.S. Challenger. At most of the stations in the deep ocean a material giving a seismic velocity of 6$\cdot $3 to 6$\cdot $6 km/s was found covered by about 0$\cdot $5 km of sediment. Some stations nearer the continents gave velocities resembling those found on land. Near volcanic islands 1$\cdot $5 to 3 km of material giving a velocity of 4 to 5 km/s was found overlying material giving 6$\cdot $5 to 7$\cdot $0 km/s.