VLF transmitter Lualualei is a facility of the United States Navy near Lualualei, Hawaii transmitting orders to submerged submarines in the very low frequency (VLF) range. VLF transmitter Lualualei, which operates under the callsign NPM on 21.4 kHz and 23.4 kHz, uses as antenna two guyed masts, each 458.11 metres (1503 feet) tall. Both masts, which were built in 1972, were not only the tallest towers in the Western hemisphere at the time, used for military purposes, they are also the tallest towers used for long wave transmissions in the Western hemisphere and since the collapse of Warsaw Radio Mast perhaps the tallest structures electrically insulated against ground.