My Grandfather was foreman of the pipe-fitting team that built Nautilus. We went on a tour after it was decommissioned, and once they figured out who he was, they pulled us from the group and let him show us EVERYthing. So I have looked in there, but I didn’t step into it.
I got to drive it several times 
And I’ve also been to Esplugues de Llobregat, which is not a mountain pass but one of the towns which form the Barcelona Metro Area.
Visited a Pharma manufacturer there, of all things. Which leads me into a trio of early 20th century gems. In order of increasing ease of access.
Building D10 at Boots, Nottingham. A living museum of a pharmaceutical factory - no public access. We were there to discuss a manufacturing contract, in a room up in the gods, overlooking the floor. Breathtaking - I’m sure they use that room just to impress the hell out of you.
The Beehive, the original Gatwick Airport terminal building - one of the classic art deco airport buildings, now used as offices. Back in the days before mobiles/cellphones, and when the Beehive was an isolated building (and wasn’t surrounded by a mess of industrial units), I had a flat and sneaked in there to borrow a phone.
Shoreham Airport (AKA, rather optimistically, Brighton City Airport), another classic art deco airport. You can eat in the restaurant there.
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As far as “places it’s highly unlikely other dopers have been”, I’ve got a bunch.
Left seat of a B-17. In flight of course (it’s in my log book).
Catapulted off the deck of the USS Carl Vinson in a C2 Greyhound.
“Drove” the SEMAC 1, a 45,000 ton (disp) 650’ semi-submersible pipelay vessel. My station was the forward operator’s position in the tower amidships.
Rafted to Monarch Cave and Wolfman Panel (Anasazi pictrographs).
Rafted Grand Canyon from Phantom Ranch to Bar 10 (5 days).
Stood out on the ramp (in the rain) to watch first flight of the 747-400 (I had worked on some of its software).
Stood out on the ramp (in the sun) to watch first flight of the F-35. (Hadn’t worked on it so no dog in that hunt. But was there for another reason and got to watch)
I’ve done a lot of amateur exploring around the site of the Battle of Marks’ Mills (Civil War). In particular, the crossing near present day New Edinburgh and Mt Elba on the Saline River. I’ve found quite a few artifacts (turned over, not kept) and I’m certain have been one of the few, or only, people to see these areas and objects for 150 years or so. One day after getting lost for several hours (there’s a local compass error) I found the apparent remains of a small settlement and even some crude building foundations. I was able to confirm those random large stones were actually purpose-placed via a laser level and measuring their near-perfect angles. They were buried in deep bush and not visible. I suspect I was the first to see these in a very long time.
If I may include something from one of my kiddos…
As part of her Archeology PhD, my daughter was working with a group (from Ukraine of all places) uncovering a shipwreck on the floor of the Black Sea earlier this year. She was operating the giant vacuum* when it uncovered a layer of ~2500 year old Amphorae. She just happened to be the operator when they came to light. She told me she had tears in her eyes realizing she was the first human to see those in 2500 years.
*Whatever that giant vacuum thing is that you see on undersea exploration shows. She’s not here so I can’t find out its exact name.
Cat Island, Bahamas.
I have also been to (or at least past the entrance to) the world’s oldest existing, functioning brewery, Weihenstephan, in Freising, outside of Munich.
Sadly I was too late to take the tour, so I found a nearby ratskeller and sampled several of their excellent brews instead.
Weihenstephan, in Freising, outside of Munich — yes, I’ve been there too! Bavarian beer, always good.
I have twice gone to the cheese factory in Gruyere, Switzerland, where they allow you to watch a cheese being made from 4000 liters of milk.
The Einstein house in Bern where he lived as a student and is now a museum.
The council chamber in Bruges, Belgium.
Probably many of you have also been to an opera in the Sydney dish drainer, I mean opera house.
A lot of great stuff here. With everything I’ve seen and every place I’ve been, there is so much more that is still unseen out there.
I’ll add a couple more:
Katmai National Park, AK, including The Valley of 10,000 Smokes, which hosted the biggest volcanic eruption of the 20th century. To get there, you fly first to the town of King Salmon, then fly by float plane to land on Naknek Lake.
Ngorongoro Crater in Tanzania, a true natural wonder of the world, IMO. This is the lodge we stayed in.
I’ve been in the cockpit of a P-51 Mustang piloted by Bob Hoover, the WW II ace and test/stunt pilot, while we did aerobatics, including snap rolls 50 feet over Lake Butte des Morts, WI.
I’ve been on a private tour of the second floor of the Ronald Reagan Library in Simi Valley, CA. This is where their private offices and reception area are located, and contained a lot of personal memorabilia. This was when Nancy was still living and we were told she visited there often to drop things off.
I wonder if anyone else ever got to tour WWVH, the time station based in western Kauai (in Hawaii). This was the mid 1980’s, and though the station is on a military site (a Navy missile range facility), they waved us through when we said we wanted to see the radio station. We went inside, interrupting one of the operators who was eating a sandwich and a bag of chips at his desk. He was happy to show us the generating equipment and point out the radio towers. He answered any questions we had. I have an electrical engineering degree, so I recognized lots of the equipment - standard HP counters that had been heavily modified for example. We could have reached out and press a “reset” button and screw up the entire time system in US!
Due to security concerns, it’s no longer available for tours.
In 1989 I spent 2 months on the Sulak, a Soviet fishing boat (really a factory ship) in the Bering Sea. Martin Cruz Smith later used it as the setting in his book Polar Star.
Has anyone been to the Dental Museum in Bainbridge, Ohio? It was the first dental school in America. Every current dental school can trace it’s lineage back to two of the original students of this first school. It’s small but worth the $5 price of admission.
Having run into people I know in the most unlikely places, I hesitate to say I’ve been somewhere that no one on this Board has been to.
I suspect very few have been to the museum in Arbroath, Scotland that shows the way they used to live, and fish, and make “Arbroath Smokies” (Smoked preserved fish from the North Sea). But I wouldn’t want to bet on it.
More than likely others have been to the Paradise Ice Caves on the slopes of Mt. Rainier, but you can’t visit them any more – they’re gone.
I’m guessing not many Dopers have been in The Greaseman’s studio audience. DC101 in Rockville, the morning before I turned 21.
I visited Moscow, West Berlin, Warsaw and Prague when they were all still cities in communist countries. Was followed by the local intelligence agencies in all of them.
I have been to 3 of the 4 of those at least once each just since early May. Cities like those are daily destinations for tens of millions of people per month.
You can do better than that.
Beckdawrek:
Was there four years ago. Didn’t find anything, but a day after our visit, someone else found the fifth-largest diamond ever found there.
burpo the wonder mutt:
Been there. There’s also a very similar place in Strasburg, PA called the Choo Choo Barn.
Bullitt:
Ditto, and also to all 10 provinces of Canada. In fact, the reason I visited the whaling museum I mentioned in my first post was purely because I wanted to be able to genuinely say I’d been to Newfoundland AND Labrador, not merely to “Newfoundland and Labrador”.
Hmm… I’ve visited Gdansk, but I’m not sure if I went there.
I’ve been behind the scenes in rather a lot of UK zoos over the years, including visiting some of the islands in the canal at Chester zoo that even most of the staff never got to.
Anyone else visited the Sultan of Brunei’s palace? Or a teeny island that I think was Panikia, in Komodo national park (may have been the next island over)?
Been there. Now the one you need to add to your bucket list is Bill’s Old Bike Barn and Museum
near Bloomsburg
Bill Morris does very little miniature and it is tons more than just motorcycles. It really is almost an indoor town.