On January 1, 1950 voice announcements were added to all broadcasts - the familiar "at the sound of the tone ...". In January 1953 the 30 and 35 MHz broadcasts were discontinued.
In June 1954, two of the four quartz crystals that made up the National Primary Frequency and Time Standard were transported to the new Boulder Laboratories where CRPL was moving. They stabilized a new experimental station - KK2XEI - operating at 60 kHz that was radiating at only 2W (precursor to WWVB).
It was picked up at Harvard and elsewhere with just 2W of radiator power. This had the potential for a global time and frequency broadcast.
So an experiment was proposed for a 20 kHz VLF broadcast west of Boulder in four mile canyon near the old mining town of Sunset. This was based on an experiment at Rugby, England at 16 kHz that was picked up in America and around the world.