Electromagnetic waves in the ELF and SLF frequency ranges (3–300 Hz) can penetrate seawater to depths of hundreds of meters, allowing signals to be sent to submarines at their operating depths. Building an ELF transmitter is a formidable challenge, as they have to work at incredibly long wavelengths: The U.S. Navy's Project ELF system, which was a variant of a larger system proposed under codename Project Sanguine, operated at 76 Hertz, the Soviet/Russian system (called ZEVS) at 82 Hertz. The latter corresponds to a wavelength of 3,656.0 kilometres. That is more than a quarter of the Earth's diameter. Obviously, the usual half-wavelength dipole antenna cannot be feasibly constructed.