The World's Most Secretive Brewery

Today's post on All About Beer is a special holiday feature.  You can attempt to guess the brewery in question:
“We do have a legendary process and there is element of mystery behind it—and you’re not going to get that out of me.”
I do have one addendum.  This is a brewery that rejected my entreaties to see the inside (the real inside, where they actually make the beer) and I wrote, "If you do much research, you discover that this has been true for a while—no one has seemed to so much as glimpsed a mash tun for years."  It turns out that's not entirely true--Brian Yaeger has been inside.  I'll have to get him drunk and find out what he knows.

Follow-up. I had forgotten this report from Martyn Cornell of a visit to the brewery organized by the European Beer Bloggers Conference (including illicitly-snapped pics of Brewhouse #4).  Nothing there makes me regret calling Guinness the world's most secretive brewery.