What is the biggest natural event (tornado, tidal wave, etc.) that was witnessed by a lot of people, but has only occured once?
Also…
Biggest natural event that was witnessed by a lot of people, but occurs very rarely (pershaps at least once in a lifetime - ie I can see it at least once)?
Biggest cosmic that was witnessed by a lot of people, but has only occured once?
and of course: Biggest cosmic event that was witnessed by a lot of people, but occurs very rarely (perhaps at least once in a lifetime)?
I don’t know about occurring only once but I would say an ice age is a pretty rare event and a pretty big one. I’m not sure, however, what state of evolution ‘people’ were in at the time so as to count as ‘witnessed by a lot of people’. You also might consider a meteor strike. While meteor showers are common enough meteors actually impacting the earth much less being witnessed are pretty rare (although they too have happened more than once).
Beyond that I’d vote for a tidal wave. While they’ve certainly occurred more than once they are pretty rare events to witness (think of all the video you’ve seen of tornados and think of the number showing a tidal wave).
As for a rare cosmic event that can be witnessed by a lot of people I’d say a supernova. Some have been visible to people without benefit of a telescope. However, some events would be even rarer such as the formation of a black hole or the collision of two galaxies. A black hole has never been viewed being formed and a collision of galaxies takes place across a huge timescale compared to a human lifetime.
A once in a lifetime cosmic event viewed by many people might be the impact of Shoemaker/Levy-9 into Jupiter.
For something everyone could probably expect to view once in a lifetime but not more (assuming a slightly longer than normal average lifespan) would be Halley’s Comet. With a period of 76 years this is a once in a lifetime event. Of course, comets aren’t all that rare but Halley’s has that once in a lifetime period going for it.
Nitpick: Someone who was 8 the first time the comet came along would be 84 the next time it came around. This is not so rare a situation. There were plenty of youngsters who saw it in 1986 who will still be around in 2061 to see it again.