Aguada transmission station is a facility of the US Navy for transmitting orders to submerged submarines near Aguada in Puerto Rico at 18°23'55"N 67°10'38"W by using radio waves in the low frequency range. Aguada transmission station, which works on 40.75 kHz with the callsign NAU, had originally 3 guyed towers. From these two were dismantled. Only the tallest with a height of 367.3 metres ( 1205 ft) is still standing and in use. It is the tallest structure in Puerto Rico.