And actually - it has 2 "M Cave" candidates which people have visited since. One - probably the wrong cave (high up, when M Cave was ground level). The other one, I feel it's a tall order that it's been "filled in". While strange, the M-Shaped cave found mostly filled with different colored sedimentary rock, it doesn't resemble any kind of man-made mortar/concrete/etc.
The military's style is to just pour high PSI concrete into a mold and call it a day, not to try to emulate natural sedimentary rock and half-ass block a cave in a way that a prybar can get past.
All in all - the real M Cave (if indeed there was one to begin with), it's appearance and approximate location is only known to someone who is not likely to ever be seen again.