Ayup 4W

Not really that bothered about the 'has', but I used it because I felt it emphasises that the hill is still in a state of collapse.

I don't think it's grammatically incorrect. If you're linking the tense of 'became' and 'has' and saying they don't match, then that's irrelevant. They describe 2 different events (becoming known and collapsing). Or maybe I'm missing something?

Comma, fair enough - that's my commaphobia showing! I hate unnecessary commas & in this case you could argue I've gone TOO FAR (LOL!!) - I'd say it's probably a matter of personal taste. We're doing 2 different things - you're parenthesising, whereas I'm just breaking the sentence. But we can stick a comma in, no probs.

love

Moth