(CNN) — An American passport opens doors.
According to the Henley Passport Index that tracks passport rankings, Americans' visa-free access to 184 countries makes a US passport the sixth most powerful passport in the world. That freedom and flexibility is what some call "passport privilege," and it might help explain the record numbers of United States citizens that traveled internationally in 2018. It's only getting easier. Even countries with historically tight controls on the visa process are inviting travelers to visit. In Saudi Arabia, a new visa program is designed to increase tourism, while Brazil has lifted visa requirements for American citizens altogether.
But red tape still entangles American travelers at a few borders across the world.
Americans must obtain a visa from a Chinese consulate before walking the Great Wall or exploring the Forbidden City. In Bhutan, a daily fee and strict guidelines keep tourist numbers low. And from Cuban beaches to UNESCO sites in North Korea, some of the world's most remarkable places can be difficult or impossible for Americans to reach. The United States government blocks its own citizens from entering the Democratic People's Republic of North Korea, while other countries set stringent rules that keep many Americans away. In Iran, the US Department of State has assigned its most severe travel advisory (official risk assessments ranked on a scale of one to four). If you're carrying an American passport, you can travel the world with a freedom that Lebanese or Nepali citizens can only dream of. (Or purchase.)
Your luck can run out at these border checkpoints.
Chonji lake or 'Heaven lake' is located in the crater of Mount Paektu, which is considered the spiritual birthplace of the Korean nation.
Ed Jones/AFP/Getty Images
Coastal plains crumple into rocky peaks in the Democratic People's Republic of Korea, the only country on earth the American government forbids US citizens from visiting.
That's when the United States government decreed that American tourists were no longer allowed to visit, citing "serious and mounting risk of arrest and long-term detention."
High-profile examples have demonstrated that risk: In 2016, American tourist Otto Warmbier was arrested and held for 17 months after allegedly stealing a political poster, and he died shortly after being returned home. Three Americans were released in May, 2018 following extended detentions in North Korea. Current laws mean the closest American tourists can get to the "Hermit Kingdom" is the Korean Demilitarized Zone, or DMZ, the fortified border between North Korea and South Korea.
What you're missing: Glimpsing a culture that's been isolated for decades, which outsiders have often experienced within the constraints of tightly controlled tours.
Historic landmarks are largely unfamiliar to international travelers. Visit the UNESCO-listed Complex of Koguryo Tombs to see brilliant wall paintings depicting a kingdom that thrived until the 7th century AD.
Cubans observe the arrival of a cruise ship bringing tourists to Havana in 2017.
Adalberto Roque/AFP/Getty Images
Havana is just over 100 miles from Key West, but the Cuban capital is beyond reach for many US citizens.
And in recent years, changing rules have caused whiplash for Americans eager to visit the nearby Caribbean island.
In the first half of 2019, they flooded in after loosened regulations made it easy to visit: According to the Cuban government, more than 140,000 US citizens had arrived by cruise ship by May of this year. In June, that parade of cruise ships came to an abrupt stop due to new rules imposed by the Trump administration. What changed? The US Embassy currently lists 12 approved categories for travel to Cuba, from journalism to humanitarian projects. A previous category for "people-to-people" travel, which accounted for many tourist visits, was eliminated. That's a blow to the nascent private travel industry in Cuba, such as the network of Airbnb hosts that blossomed under relaxed rules introduced by the Obama administration. What you're missing: Exploring Cuba's 3,570 miles of coastline, which includes white-sand beaches prowled by baby sea turtles. In the charismatic capital of Havana, a trove of Art Deco landmarks preserve the style and architecture of the 1920s and 1930s.
CNN's Fred Pleitgen visits the Iranian city of Shiraz, near the ancient city of Persepolis, to explore Iran's growing tourism business.
Add a tradition of hospitality and fine cuisine for a traveler's dream destination.
Part of the reason may be the difficulty of obtaining a visa to Iran. Americans visiting Iran are required to travel with an officially sanctioned host at all times, and red tape can mean unpredictable delays.
While many American tourists get approval within a few weeks, the guiding company Intrepid Travel suggests that US Citizens allow for 60 to 90 days to get a visa.
An unstable political relationship between the United States and Iran adds additional uncertainty: Following President Trump's 2017 executive order limiting immigration from Iran and six other Muslim-majority countries, Iranian officials announced they would ban American visitors in retaliation.
US Citizens are welcome in Iran for now, but political ties remain fraught.
Mount Damavand is Iran's highest peak and a potentially active stratovolcano.
Atta Kenare/AFP/Getty Images
That's not all: Iran is designated with the US State Department's top-level travel advisory, and once there, Americans are required to travel with a officially sanctioned host at all times. In Iranian Kurdistan, winding roads lead to the magnificent Hawraman Valley, where houses are built into steep hillsides.
The Dutar monument (front) symbolizes inspiration in Turkmen, with the Turkmen State Institute of Culture in the back.
Stanislav Krasilnikov/TASS/Getty Images
This desert region might have been a travelers' hub in the ancient world, but these days, Turkmenistan imposes visa rules that keep many visitors away. To get a visa to Turkmenistan, Americans need an official letter of invitation from the government; arrive without one, and you could be held at the airport until you book a flight out of the country.
For most travelers, that means visiting as part of an organized tour, as companies can arrange letters of invitation that make access easier.
A day trip away from Ashgabat is Darvaza Crater, a collapsed, flaming gas cavern known as the "Gates of Hell."
Moscow is filled with historic, significant structures, inclduing St Basil's Cathedral.
Alexander Nemenov/AFP/Getty Images
Yards of red tape lie between American travelers and a trip to Moscow's iconic Red Square, where visitors can still visit the preserved body of Vladimir Lenin.
Show up without a visa, and there's no way in.
To obtain a visa in advance, travelers from the United States must get a letter of invitation. (Most visitors work with a tour agency that can confirm plans and issue the letter.)
Tourist visas are issued for a maximum of 30 days. While that's not unusual, Russia's harsh enforcement of visa regulations is. Overstay your visa by accident, and you'll need to apply for a visa to leave the country.
According to the US Department of State, travelers attempting to leave Russia with an expired visa can be held for up to 20 days while waiting for an exit visa to be issued.
Other highlights? Luxuriating in a traditional Russian bath, or banya, and pairing the country's hearty cuisine with endless rounds of vodka shots.