As Turkey's biggest city and most visited tourist destination, Istanbul is a sprawling network of streets, neighbourhoods, and buildings. Most people are already aware of its historical story as the capital-ruling centre for both the Byzantine and Ottoman empires, but we searched high and low to find the most unusual facts that the mainstream travel brochures fail to mention, and this is what we found. How many of these facts did you already know?

1 It was known as the City of Seven Hills

Following in the footsteps of imperial Rome, Istanbul was also known as the “City of Seven Hills.” All hills were within the old boundary walls, and the first was where construction of Byzantium started. That location is the historic Sultanahmet area where today the Hagia Sophia and Blue Mosque stand. When the Ottomans invaded, they built an imperial mosque on the top of each hill.

2 It's not the capital city

Istanbul is NOT the capital of Turkey, but is the largest city so naturally, it also has the most mosques, in total 3113 of which the Blue Mosque is the most famous. Dating from 1616, it uniquely has six minarets and is one of only two mosques in the country built in this style.

3. It's the birthplace of Murder on the Orient Express

The celebrated author, Agatha Kristie took residence in the famous Pera hotel while she wrote, her famous novel, called Murder on the Orient Express. Within the book, riding the historical train line of the Orient Express running from Istanbul to Paris, the character Hercule Poirot famously solves a murder that happens on the train.

4 It's home to the spoon-maker's diamond

Istanbul is home to the world's fourth largest diamond. The Spoon-makers diamond, held within high-security conditions at the Topkapi Museum is a beautiful and stunning jewel admired by many. The real story of how the Topkapi Palace acquired it is unknown although local legends say a fisherman found it on the shoreline and unaware of the treasure he held, sold it to a jeweller, for three spoons, hence the name. He then sold it to Ottoman royalty.

5 ... and the relics of a prophet

The Topkapi Palace in Istanbul is also home to the relics of Muhammad, the greatest prophet of Islam. Stored in the section called the Chamber of the Holy Mantle, the Ottoman Sultan Selim first acquired them in 1512 and then Mehmed the 3rd bought them to the Topkapi Palace in 1595.

6 It's home to the country's first eco-skyscraper

The tallest building in Turkey is in the business district of Levant and it is also the country's first ecological skyscraper. At the time of construction in 2010, it was the fourth largest building in Europe with a total height of 238 meters above ground level.

7 Mark Twain wasn't a fan

Mark Twain hated his visit to Constantinople in 1867. Referring to what is now, some of the greatest touristic sites in the city, he called the Grand Bazaar, "monstrous hive of little shops.” He referred to the Hagia Sophia as the "rustiest old barn in heathendom" and said the ancient cultural practise of a Turkish bath was a "fraud."

8 It's home to the third-oldest subway in the world

The underground funicular railway system running between the districts of Karakoy and Beyoglu is the third oldest subway in the world. Built in 1875, it measures 554 metres long and originally consisted of two wooden car trams, although these have now been replaced taking the travel time to just 1.5 minutes.

9 There are a lot of cemetaries

Istanbul has a total of 333 cemeteries of which 268 are for Muslims and the rest for other denominations such as Christians and Jews. As well as its significant Christian history, the city has had a strong history of Jewish neighbourhoods ever since the conquest of Constantinople in 1453 when Sultan Mehmed the 2nd encouraged people to move there to repopulate it. Districts like Galata and Balat are primarily known for their Jewish history.

10 It has a few pseudonyms

Sometimes called the city of three names because of its history of Istanbul, Constantinople and Byzantium, other lesser known names include Lygos, its first name and New Rome, called that by Constantine the Great before he made the city, the eastern capital of the Roman empire. In the 19th and 20th centuries, the western world also referred to it as Stamboul, but this specifically applied to the historical peninsula that these days include the UNESCO World Heritage sites in the Sultanahmet district.

11 It's enduringly popular

In 2018, the MasterCard Global Destination Cities Index revealed that Istanbul was the ninth most visited city in the world. Its location on two continents was partly credited for its popularity and following closely behind Tokyo; it even beat Barcelona, Rome, and Milan.