The Morrison Knudsen Tunnels are located in the San Rafael Swell area of south central Utah. I first heard about this site in 1997 and it took some digging to figure out the exact location. At that time, there was no published material and the only internet reference was a cryptic posting that mentioned a military "thing" with a massive blast tunnel near the "Wedge Overlook". Foolishly, I assumed that the Wedge Overlook was the ridge that overlooked the Utah Launch Complex. I combed the area around the complex for hours searching for some hidden tunnel. After several inquiries, I eventually found a BLM employee who was kind enough to send me a map with an X marking the spot....roughly 40 miles from where I had been searching.
From the Sun Advocate:
In 1948, when the United States Department of Defense was looking for naturally occurring defenses from air-delivered explosives, its search brought it to the Western flank of the San Rafael Swell. Several horizontal shafts were created, after which varying amounts of explosives were detonated above them at ground level. The relatively soft rock of the Navajo Sandstone probably didn't perform as well as the hard granite of Colorado where NORAD and other military facilities were ultimately located. There were no military installations developed here. However, the craters that resulted from the detonations, along with the prepared shafts, gained some local notoriety and have come to be known as the "MK Tunnels," taking their name from Morrison Knudsen, the contractors DOD enlisted to prepare the shafts.
Also from the Sun Advocate:
According to the records that were released later, blasts were set off ranging from a number of charges with 320 lbs. of explosives to the biggest blast that took place on or around Oct. 4, 1948 that employed 320,000 lbs. of explosives.