It's ceased to be underground twice (as mentioned previously) 74/5 with the likes of Pye disco demand, TOTP appearances etc then again in the mid 90s with a massive influx of returnees plus a lot of never were's of the same generation. Factors second time - TV adverts, cd compilations available in your local supermarket, the internet and large promoting organisations who's main concern is getting the maximum number of bodies through the door.
For an underground scene, its only been underground for about 40% of its total life approx 20 years out of 50 (65-74 and 79-95)
Of course there's still a true underground going to this day that the majority are either unaware of vaguely aware of, but thats another topic...