What's the most fascinating structure or object you've been inside of?

The liminal stages around Taj Mahal

Based on size? Spaceship Earth in EPCOT Center.

On a more personal scale, a lighthouse.

gives OP a hug It’s okay, they mean well.

A nuclear reactor would be at the top of the list, but it wasn't as fascinating as Dutch windmill when I was 6 years old.

When first I learned of the tower at Bok Tower gardens I thought, how ostentatious of a man to build himself a tower. But then I was given the rare privilege of standing at it's precipice. I get it now. I felt like a wise wizard, like Saruman, looking down at the world. If ever I am able I must have a tower, if only to drink my morning coffee and wonder from above at the majesty of the world beneath me.

I also played a short melody on the bells, from house of the rising sun, before being asked to stop touching everything.

It was a long time ago, so my memory may not be exact, so I'll try my best to describe what I saw, also I was not allowed into every room, but I did sneak away and take a peek behind a door, it was a broom closet.

The inside had very intrically carved antique furniture, dark brown, maybe mahogany. I was not allowed to open the cupboards, I did, it was empty. Blue or teal mosaic tiling on the floor, a white marble bathroom tucked away in the corner with I believe golden fixtures. The metal staircase wound up along the sides into the reaches of a Gothic vaulted ceiling. It didn't feel like a church, it felt more like being in a Freemason temple. Oh the front door is golden or brass and I believe there is a grave at the front as well.

The next floor contained the Gardens archives. I was not allowed in and only saw a dim and lonely room with desk and lots of papers or books. I wish I could have spent several days in there rifling around.

Next up was the old cisterns that used to be used to collect rain water from the roof and water the gardens. They look like giant steampunk iron containers. Not much in there but assorted boxes and such. I wanted to bang on the containers, but tempered myself.

I do not remember the next floor very well, I think there was a mildly modern office, with a windowed door? I think remember seeing the American flag, but the memory is hazy.

Next up was the library. I was excited when I heard the word library, and disappointed to learn it was the largest library of Bell ringing music (I don't know the proper term for 'Bell ringing music'). There were also some books and such, but wasn't as interesting looking as I had hoped, and we didn't spend time there.

I do not remember the next floor at all, perhaps because it was unremarkable or because the next floor was where I really wanted to be, the very top.

Standing at the balcony was breathtaking. The bells were to there and of course they were a sight to themselves, but the view from the edge was...inspiring. I wanted to be able to stand up there every day, every sunrise and sunset. The sun was near to setting by that time, as we were touring after hours, so the lighting was perfect. I could see the houses and trees and fields stretched far into the distance. I didn't want it to end. I needed a staff! I needed a throng to command or a people to assure, or even better, a quiet life locked in my tower studying the mysteries.

The world needs more towers. I do not understand why today's rich have not built themselves great and beautiful towers for study and art instead of ugly sprawling resorts for decadence and indulgences. Every city and town should have one for the people as well. We could use some perspective, and inspiration.

Bring back beautiful towers!

And wizards!

Sagrada familia in Barcelona. Genuinely jaw dropping and nothing else has topped it in my travels.

OP's mom.

There was a period where I regularly got to go inside Boeing’s Everett factory for work (I didn’t work for Boeing though). For those who don’t know, it’s one of the largest buildings in the world, built in the 60s to manufacture 747s. Now they build all kinds of aircraft there.

“Big” is an understatement. Even “cavernous” falls short. It’s easy for your brain to forget you’re in an indoor space until you look up and see a roof over your head. It’s like a miniature city in there. It’s got its own road network, fire department, cafeterias, and I heard it can even have its own weather.

My route to and from the job site every day took me through alleyways and around sites where workers were actively putting airplanes together. I got to watch an entire fuselage be moved from one side of the factory to the other by the overhead cranes. But my favorite part of the whole place were the underground tunnels that you could use to get around. You could still see old civil defense fallout shelter signs in the stairwells, and even though I wasn’t supposed to take pictures in the facility I did anyway:

I went inside a collins class submarine. I went up one on those giant stadium lighting towers. I participated in an emergency exercise walking through train tunnels under a river and egressing at the escape and ventillation tower. I went inside an abandoned brick kiln, like a big oven that you could park a mack truck in. I went up inside the boiler of an abandoned coal power station furnace. But by far the biggest object Ive been inside of is OPs mum.

Inside the fresnel lens of a lighthouse. (The tallest land-based lighthouse in the UK)

Your mom!

But seriously, a pyramid at Giza. Pretty warm in there after 3500 years in the sun.

The Scott Monument, it's a gothic religious looking building in Edinburgh, but it's dedicated to the writer Sir Walter Scott.

There are many questionable practices in museum work, and of course the Lourve might be the one of the worst when it comes to that. But I have never been to place with sooooo many well preserved sculptures with super thorough explanations. I LOVE sculpture, especially the ones made with less than forgivable media for making mistakes. The Three Graces is absolutely captivating.

I sat back to back with this cute girl at a music festival (first date) strategically placed in the center of several gongs. The artists began gently rolling the gongs in succession in the middle of this electric forest and then they worked together to create rhythms and move them around us.

It is one my favorite memories of my wife :)

The Cologne Cathedral.

It took 500 years to build it.

That is so many generations. Imagine you are just the 4. architect. Your great grandfather has started it, but you did not know this man. Somehow the plans have been passed down to you, but of course there were changes... Somebody after you is going to finish it. You do not know how it's going to look in the end, because there will be more changes later. If they will be able to finish it at all....

A neutron reactor (Institut Laue-Langevin). The mix between 80's concrete bunker like structure or yellow steel bars and uber high tech mirrors that reflect neutrons is amazing, as is the crudeness of the experiments that achieve the best matter probing on earth. It's straight out of retro-futuristic sci-fi, and at the same time higher tech than many modern sci-fi will imagine in some technical areas.

I got to tour Bruce Nuclear. I still remember entering that building and feeling like I was on the Death Star with so much machinery stretching on and on.

A chip fab. Highest level clean room at the time. Amazing experience.

Sagrada Família is up there - at least for artificial structures. Amazing architecture.

Hagia Sophia Grand Mosque in Istanbul.

Built in 537 by the late Romans and still standing even though it's in a relatively active earthquake zone, it's amazing it's been around for 1500 years without collapsing. The central dome is so huge and so high, it's amazing to think how they built it back then without modern machinery or technology.

Back in the late 90s, in the CA East Bay, one of my family’s neighbors was a big shot (director or something, can’t recall) at LLNL’s National Ignition Facility (at the time, it was still called the NOVA laser, I think). My dad got this guy to give us a behind the scenes tour (including clean suit sections) of the complex, including the target chamber where they did inertial confinement fusion experiments (read: shot really fucking powerful lasers with support machinery the size of several contiguous Costcos smashed together at a tiny little gold cylinder with tritium suspended in it), and I got to stick my head in the inspection port.

It was super awesome, and one of the things I credit for making me go down the STEM track in the first place. Also, this was pre-9/11, and in the “peace dividend” era, and I’m fairly certain there’s precisely zero chance a random neighbor kid would be allowed backstage like that in such a sensitive (technically, as well as national security) area these days.

Also, I got to wave at the normie tour group from the other side of the tour glass while in part of the laser hall with our clean suits lol

There's not much left of the Mer de Glace glacier, but every year they still carve a giant ice tunnel into it so you can walk into a glacier. Pretty fascinating

The steam tunnel system under that big midwestern university. Once in, it led underneath most every building on the campus. There are many other mysteries hidden in the underground -everywhere-.

Singapore Changi Airport. Huge garden and waterfall at the centre, electric train to commute between sections.

Angkor Wat is very cool, remarkably well preserved, beautiful and a huge area of ornately carved buildings and a palace to walk around in.

West Kennet Long Barrow is a Neolithic barrow over 5,500 years old. It's part of the numerous neolithic structures in Wiltshire, UK (which includes the frankly far less impressive but more famous Stonehenge).

Going inside it is a very odd feeling. You can see and touch ancient work marks put there by people who are so remote to us we know very little about them. I've visited numerous ancient world heritage sites and its unique (to me) in how close you feel to those people.

Hot air balloon ride wit ma SO,

Safest flight time was in the early AM so we got an amazing sunrise view out of it!

An ancient Buddhist temple in Kyoto. I don't recall its name without looking it up.... But it was much less crowded than most of the other temples and shrines in the area.

I got to sit in one of the Batman Begins tumblers that was used for filming the movie(s). I used to work where I had easy access to one and early one morning when nobody was around I figured out how to open the door (hatch really over the driver seat) and climbed in. How cool it looked on the outside was unfortunately inversely proportional to how comfortable it was on the inside. Picture a NASCAR car and you’re most of the way there. Neat experience

Oh and also OP’s mom…

The pyramid of Khafre in Giza. It's the one with still some of the cladding remaining on top. The second largest one of the three.

SS Thistlegorm was really neat. It is a British ship that was sunk by the Germans in WW2. It lays off the coast of Egypt, and is now a place where you can go scuba dive inside of. It still has all kinds of cars, trucks, and motorcycles inside of.

Santa Maria del Fiore. Yep, the cathedral from Assassin’s Creed 2 in Firenze.

The CERN LHC in Geneva.

I've been inside a giant red wood. With a car.

I got to dangle outside the highest window on the Chrysler building. I took a photo looking down that shows the eagles at the top, but can't for the life of me find it :(

I had access to the real life equivalent of the Dr. Doofenshmirtz tower.

Probably not even personally most fascinating and doesn't compare to other examples here, but I was recently in the atrium of the Atlanta Marriott Marquis and it's amazing. Vast tall space with repeating brutalist architectural elements.

I can't seem to upload pics here, check it out on google maps.

The Blue Grotto in Capri, or maybe some of the less famous grottos on the Mediterranean Coast. There is something so terrifying about being surrounded by rock formations, I'm not typically a claustrophobic person, but felt a visceral fear of being under that much rock with waves moving in and out of the cave. And yet the grotto is gorgeous because of the reflection of the light. The endless dark of the cave is lit by the most brilliant blue of the water.

I like to imagine the person who first discovers something like that, having to overcome the fear of the unknown and be rewarded with the best mother nature has to offer.

Borobudur Temple

I swam to the bottom of the first chute st Jacobs Well. The cenotes in Tulum Mexico were also pretty cool.

Taj mahal, Grand canyon, and those spinning things at the carnival that look like a UFO

I don't think I've ever been to any incredible landmarks/buildings etc. So I'm gonna say, a glider.