The advantages of utilizing the extremely low frequency (ELF) band for communications to submarines are principally the low attenuation rate of ELF signals in seawater and the low attenuation rate in the earth-ionospheric waveguide. ELF transmitters, however, require high input power. Efficient signaling schemes and sophisticated receiver designs help minimize the required transmitter power and hence system cost. One such signaling schemeconvolutional encoding and binary antipodal modulation-and an experimental receiver were successfully field tested and are described. Tests included real-time reception of messages aboard a submerged submarine in the Atlantic Ocean. It is concluded that the receiver, which included adaptive nonlinear noise processing, notch filtering, ocean channel compensation, and sequential decoding makes extremely efficient usage of available signal strength and demonstrates the technical feasibility of communication in the ELF band.