Fortress in the Amazon???
There have probably been others, but here are the ones I can recall off the top of my head:
Golden-Age Superman: Secret Mountain Citadel;
Silver-& Bronze-Age Superman: Fortress of Solitude constructed inside a mountain in the Arctic; Undersea Fortress;
Iron-Age Superman: Underground Fortress of Solitude in the Antarctic (mid Iron-Age); Fortress-in-a-hyper-tesseract-sphere (aka T.A.R.D.I.S. because it's bigger on the inside than it is on the outside - late Iron-Age; constructed by John Irons) in the mountains of the Amazon;
Post-IC Superman: Fortress back in the Arctic, but now grown from a Kryptonian crystal.
I also seem to recall something about a Fortress in the center of the Sun, but that may have just been a Grant Morrison moment.
As far as what metal to name this new Age after, I've been giving the matter much thought. I believe a pronouncement shall be coming soon.