1. Leavenworth

A mysterious small set of abandoned storefronts was recently discovered beneath the small Kansas town of Leavenworth. No one really knows what the purpose of this underground establishment truly was - whether it was used by escaped slaves or part of a forgotten underground town that the new Leavenworth was built on.

2. Petra

Petra still holds some of the most beautiful ruins in the world. Taken into Roman rule around 103 AD, the city's main water system was damaged, and it was eventually abandoned by its inhabitants. All knowledge of Petra was lost to the Western world for more than 1000 years, although its stunning ruins, a blend of Greek, Roman and Nabataean architecture, can still be visited by tourists today.

3. Thonis-Heracleion

Forgotten and hidden underwater for more than 1200 years, Thonis (or Heracleion, as the Ancient Greeks called it) was once a vibrant city and one of the most important destinations of the Mediterranean. Artifacts such as ancient Greek tablets, temples and sculptures that have remained strangely intact and more than 60 shipwrecks around the city have slowly uncovered the tale of the busy port town of Thonis-Heracleion, also leading experts to believe it had sunk after an earthquake.

4. Mohenjo-Daro in Pakistan

The Indus Valley Civilization is considered to be one of the oldest ever, its peak having been reached around the year 2000 BC. That civilization is also the home of a mysterious forgotten city known as Monhenjo-Daro in modern day Pakistan. The curious thing about this city is the lack of any imposing palaces, which has led some to believe its inhabitants were egalitarians. It is not known why the city was abandoned around 1800 BC, but it was only rediscovered thousands of years later, in 1922.

5. Phanagoria

Phanagoria is a lost city of Ancient Greece partially buried beneath sand and water on the Russian side of the Black Sea. Until it was rediscovered in the 18th century, the city was largely a myth. Even today, it's unclear why it was abandoned in the 10th century, after having been active for over 1500 years.

6. Timgad in Algeria

Timgad was erected by emperor Trajan in what is known today as the Sahara desert. Surviving the upheaval of the Roman Empire, the city thrived for centuries as an important trading center. In the 7th century AD, however, Timgad suffered its second sacking by vandals, and it was soon abandoned, remaining buried and preserved by the desert for more than 1000 years. It was finally rediscovered in 1881.

7. Tikal

Tikal used to be the capital of the Mayan kingdom and one of the greatest cities in the New World. It was considered to be mysteriously abandoned and then forgotten by the local population. As the jungle grew all around it, hiding it from view, centuries passed until an expedition managed to rediscover its impressive royal palaces and 70 meter high pyramids in 1848.

Without a doubt, taking a look at the ruins of these remarkable ancient cities would make for an unforgettable experience. Do you know about any other forgotten cities I might have missed?