Back in the middle-late 1970s, I attended one of the first USENIX conferences, held at Columbia University in NYC. This was not a fancy affair, a guy from the Univ. of Texas and I shared a dorm room for housing during the event. In our spare time, we decided to see the World Trade Center. At that point, both towers were essentially complete, but (I believe) the massive antenna array on the North Tower was still under construction, at least partly. The observation facilities were in the South Tower. We took the escalator up a floor or two, paid a small free, and rode the express elevator -- no car changes -- to the top. The outside observation deck was closed due to wind, but an entire floor just below was the inside observation area. You don't realize until you're in there how closely spaced the exterior beams were -- and so how narrow were the glass windows, that had decals on them identifying the various sights.

Two events occurred on that visit that still give me chills today. First, I made the offhand comment that I didn't see how the building would hold up if the exterior framework were breached -- that's all there was, the exterior frame and the inner core -- no support pillars. I said something like, "If anything happened to the outer frame, I don't see how this building wouldn't collapse like a stack of records on a record changer (look it up, kids). The other event. Suddenly there was a bell ringing or buzzer buzzing -- I don't remember which, and the entire wide area of the floor was closed off by a massive steel barrier. I had no idea where it came from, and neither did anyone else. We seemed to be trapped. Nobody had a clue what to do. It was, frankly, damned scary. It turned out that there was a false fire alarm, and those massive barriers were automatic fire doors. Eventually the fire department came, the doors were opened, and we were all able to take the elevator back down.

That was my one visit to the World Trade Center towers. All these years later, I still occasionally have nightmares about it.