I found out just now it is down due to lack of funding. The mention of it on FB states it should be up later this year.
That's a shame. Certainly hope that's correct and they will be back. It is such an inspiring resource.
I did find one mirror site, but some of the links were broken. I didn't check the others.
Quite a bit of the material does seem to be preserved on the Wayback Machine archive site, https://web.archive.org/ . I've not tried checking all the links, but those I tried seemed to be OK, and you could still download the PDFs, and such. Not sure if the Flash Atlas will still work properly though (is Macromedia Flash Player 10 still a thing under Win 10?), as none of the "Launch this site in the KV Atlas" links I tried would function via the archived site.