Corona was one of the last undergrounds built in the Bell System, circa 1970.  It was supposed to be a hardened terminus for a never-built coax system that was supposed to stretch to Julian, CA to the south.  Although quite large, Corona was a single story facility.  It was envisioned that a hardened "beltway" would be built around the metropolian Los Angeles/Orange County area of southern California, with Mojave to the north and Corona to the south.  The thought by this time was that in the event of a full-scale nuclear attack, Los Angeles would be a total loss, along with all communications facilities within it.  Essential communications would be transported around the "smoking hole" of Los Angeles from Mojave to Corona, and thence south to Julian's AUTOVON, Echo Fox, and other necessary systems.  There was talk of an "L-6" system (never deployed) being used for this Corona-Julian leg.  Of course, by the time Corona was finished, the futility of "hardened" communications was finally realized, and all further construction stopped.

As it came to be, Corona was a southern terminus for a non-hardened L-4 "backbone" system that went from there to Newhall Jct., north of Los Angeles.  Pacific Telephone utilized this backbone more as a combination short-haul and long-haul carrier, with very short (comparatively) sections between Newhall Jct., Sherman Oaks, Los Angeles 03, Gardena, Anaheim, and Corona.  Complete terminal facilities were provided at each of these offices, and most interoffice toll traffic rode this route.  In 1979, the last L-carrier system ever built, an L-5E, was installed along this route to add much-needed capacity in Southern California, relieving overcrowding on microwave radio radio routes and the L-4, and providing exceptionally quiet service at the same time.  By 1986, the L-5E was gone, replaced by Philip P-565, and within four years, even that was gone.  The cables for the L-4 and L-5E were both scrapped a few years later.

AUTOVON traffic destined for Julian was routed over the L-4 from Mojave to Los Angeles, via the "semi-hardened" L-3I, then to Corona via non-hardened L-4, and then to Julian via TD-3 microwave radio.  The hardened "L-6" was never built, nor does Julian function today.  Last I knew, Corona was just an oversized "manhole" for a lightguide regenerator.  Plans in install a
new 4ESS dedicated to local access and cellular traffic never materialized as long as I was there, although there may be something there now.  At one time, the huge amount of extra space in Corona was utilized as a SCOTS center ("Surveillence and Control Of Telecommunications Systems", remote monitoring and control of microwave sites), and, after the  normalization" of Pacific Telephone toll facility sites in California by AT&T Long Lines, was upgraded to an FMG ("Facility Management Group", more of the same, but with lots of white shirts calling shots).

Typical of AT&T blundering, the whole FMG was moved from Corona underground to the basement of the San Bernardino Toll Office, where the basement actually sat in the high water table, and the office sat over an earthquake fault.  After figuring that out, AT&T closed the San Bernardino FMG and moved its function to Denver, which was also part of a plan to remove all
critical functions from California.  Today, little of the formerly large presence of AT&T remains in Southern California, what with many remote sites closed and gutted.  Large offices full of sales and management types also closed.  Only a handful of communicatinos technicians remain there, no operators, no sales staff, and very few managers.  A long-running joke of
telephone people out here is that the New Jersey managers of AT&T regard California as still being "cowboys and Indians" country.  This stale thinking caused MCI and Sprint to flourish out west, much to AT&T's surprise.
 
 

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