It doesn't actually. That's a proxy, not a mirror; designed to make TPB available from countries where it's blocked. Try a search, it won't work.

https://proxybay.info/

However, there's a script where you could download the whole TPB, it's only 90 Mb. That's possible because now they're using only magnet links, so a torrent is pretty much just a URL.

http://torrentfreak.com/download-a-copy-of-the-pirate-bay-it...

So if someone have a recent copy, it would be easy to setup a mirror.

Or people could just use one of the dozen torrent site still up and let the whack-a-mole game continue.

A while ago someone from TPB guys explained their setup. Their web-facing webservers connect to the actual backends via a VPN that does some fancy routing to hide the real location. On top of that, they have a ton of mirrors and proxies running.

It's called Whack-a-mole. Anyone waiting for Pirate Bay to inevitably come back up can use:

http://www.torrentz.eu

http://www.kickass.to

etc, etc.

Proxies to mirrors.