Is there a way to regenerate maps that I've exported and closed the tab on?
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You can't import back an exported map, but you can open that exported json file in a text editor or in a browser and locate a line which looks like this:
"origin": "https://watabou.github.io/perilous-shores/?seed=1408185754..."
That url is a link to your original map. "Original" means that it doesn't include any changes if you did any.