I found this, the other day, and thought I'd share. Truthfully, imho, there are many more sites than the ones listed.

~NightSky

PS Bonus places: Mausoleum of the First Emperor - Qin Shi Huang (with Terra Cotta Army, in China, the Lascaux Caves in France, Fort Knox Vault (Is the Gold still there?) Surtsey Island, Iceland, North Brother Island, USA, Dulce Base, USA, Niihau Island, Hawaii - USA (Only the US Navy can go here, or with express permission)

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By Benjamin Taub

Area 51 isn’t the only place that’s kept under wraps. In fact, the world is full of closely guarded sites that no one is allowed to enter, either for their own safety or to protect a secret. Here’s a look at five of the most enigmatic spots that no one is allowed to enter.
North Sentinel Island

Located in the Bay of Bengal, North Sentinel Island is home to the world’s most isolated group of people. One of the few remaining uncontacted tribes, the Sentinelese are fiercely hostile to outsiders and guard their small, jungle-covered island with deadly force.

Roughly the same size as Manhattan, North Sentinel is perhaps the most famous of the Andaman Islands, having gained notoriety as a result of several disastrous and ill-advised attempts to visit the paradisiacal spot. The most recent of these occurred in 2018, when an American missionary was killed by the Sentinelese while approaching their homeland, apparently with the intention of converting them to Christianity.

In truth, the Sentinelese are wise to resist visitors so fiercely, since the introduction of viruses and other pathogens by Westerners could easily wipe out an isolated tribe that lacks immunity to these illnesses. For the protection of everybody, therefore, the Indian Coast Guard now patrols the waters of the Bay of Bengal, enforcing a strict ban on all visits to North Sentinel.

The Vatican’s Secret Vault

Founded in the early 17th century, the Vatican Secret Archive - which was renamed the Vatican Apostolic Archive in 2019 - was built to house the personal documents of all the popes. For more than 250 years, these records remained completely sealed off, but in 1881 Pope Leo XIII finally relaxed certain restrictions, allowing a select group of Catholic scholars to access the vault.

To this day, however, the archive remains off-limits to all tourists, with journalists also barred from entering. Only a very small number of carefully accredited religious academics are allowed in, and even they are forbidden from browsing the collection.
The Doomsday Vault

Officially known as the Svalbard Global Seed Vault, this protected facility is located on the island of Spitsbergen - the northernmost permanently inhabited place on Earth. Deep within the Arctic Circle, the popularly named Doomsday Vault contains seeds from over 1.3 million plant and crop species from around the world, providing a vital back-up in the event that some natural (or, indeed, unnatural) disaster should wipe out our existing supplies.

Carved into bare rock, the facility sits more than 100 meters (328 feet) inside a mountain and has been built to withstand earthquakes, nuclear attacks, and rising sea levels. It’s also heavily protected against germs and other unwanted visitors, including humans.
Snake Island

Ilha da Queimada Grande - also known as Snake Island - became cut off from the Brazilian mainland around 11,000 years ago, at which point the local snakes embarked on a considerably wilder evolutionary path than their continental counterparts. Eventually becoming what we now know as golden lancehead vipers, these incredibly dangerous snakes possess a venom that can melt human flesh, resulting in death within an hour.

According to legend, the island’s last permanent inhabitant was a lighthouse keeper, whose entire family is said to have been killed by their serpentine neighbors back in the 1920s. Since then, the snakes have completely taken over, with some reports suggesting there is now one snake in each square meter of the island.

Given the dangers posed by the local wildlife, no one in their right mind would want to visit Ilha da Queimada Grande - even if they were allowed to. For obvious reasons, then, the Brazilian government has banned anyone from setting foot on Snake Island, although some researchers are allowed to study the snakes, provided they are accompanied by a doctor.
The Church of St. Mary of Zion

Indiana Jones may have found his Holy Grail at Petra, yet legend has it that the Ark of the Covenant is actually housed within a church in Aksum, Ethiopia. As in the film, the guardians of this religious relic are said to be forbidden from ever leaving its side and are supposedly trained to kill anyone attempting to enter with their bare hands.

In reality, the revered object found within the Church of Our Lady Mary of Zion is likely to be a replica of the Ark, which may or may not actually exist. However, as no one currently alive has been granted permission to enter the church, it’s difficult to know for certain exactly what it contains.