9/11 for me without a shadow of a doubt. I was working at a radio station at the time and will never forget how we heard rumours of a small plane accidentally crashing into something in New York to it being a full on major event. Watching it on the TV as it happened was like nothing else and I genuinely felt scared as my Mum and Dad were on holiday in the Canaries. If you had told me it was aliens invading earth, I'd have believed you in that day. The world changed that day, no question.
Chernobyl - I was only a kid at the time, but the fact something so far away in Russia was causing problems in Wales scared me. I remember that breaking and getting the fear that there was a silent, invisible 'killer' around and it felt very scary at the time. I remember stupid things like my Mum getting a "bargain" on Oldham market of half a lamb for pennies and my Dad going mad when she brought it home and he wouldn't even have it in the bin and went to the tip with it as he was scared about radiation from it (it was a Welsh lamb).
Covid in Italy. Back in January(?) 2020 I saw a video of what looked like a military convoy going through Bolsano and someone said they were taking bodies out of hospital that had died of this new virus. I dismissed it at the time as "fake news" and wasn't worried about this new virus as we'd heard about them and they never affected us. I remember seeing it again and the dawning fear that came on me that this was real, was coming and I was completely unprepared for it. Soon after, a friend of mine in Madrid sent me a video of people locked in their apartment blocks and just getting the chills, it shook me to the core as it felt so... alien.