In 1942, contractors for the U.S. Navy began construction of the Haʻikū Radio Station, a top secret facility that was to be used to transmit radio signals to Navy ships that were then operating throughout the Pacific.In order to obtain the necessary height for the antennae, the Navy stretched them across Haʻikū Valley, a natural amphitheater surrounded by high ridges. To accomplish this, they needed “easy” access to the top of the ridges, so they installed a wooden ladder up the mountain. The ladder was later replaced by a wooden staircase. Once the cable car was in operation, most workers preferred to ride the car to the upper hoist house rather than enduring the tedious climb up the stairs. Some remnant parts of the wooden ladder may still be seen beside the metal steps.
The radio station was commissioned in 1943. To transmit such a powerful signal, the Navy needed a transmitter of greater capability that was then possible with vacuum tube technology. They therefore decided upon an Alexanderson alternator, a huge device capable of generating powerful low-frequency radio signals, and requiring an antenna of heroic proportions.
When the Naval base was decommissioned in the 1950s, the U.S. Coast Guard used the site for an Omega Navigation System station. In the mid-1950s, the wooden stairs were replaced by sections of metal steps and ramps — by one count, 3,922 steps. The station and trail were closed to the public in 1987. Some hikers ignored the "no trespassing" signs and continued to climb, contributing to the local community's misgivings about reopening the structure.
According to wikipedia