Bunker popularity reached fever pitch during WW1 and WW2 for obvious reasons. Over time a number were abandon, stripped and forgotten. Some however were developed further. Now, in recent times there seems to be a resurgence.
TIME Magazine had this to say:
Multiple manufacturers throughout America are reporting 20% to 1,000% increases in inquiries and sales following the March 11 [Earthquake] disaster that left thousands dead and missing in Japan, and it appears that people are willing to pay a hefty price to keep themselves and their families unscathed in the impending apocalypse.