CRUMBLING walls. Breathtaking temples. Mysterious cities built entirely underground.

They are the astounding feats of architecture that have been left to decay for centuries.

But while they may be in ruins, the sites of the world's most ancient and intriguing cities continue to wow travellers.

From the popular Machu Picchu site in Peru, to the Pompeii ruins and the lesser-known Derinkuyu site in Turkey, here are eight amazing ruined cities that remain shrouded in mystery - or remain perplexing to this day, according to the science website io9.com.

One thing's for sure, the world is a fascinating place.

1. Palenque, Mexico

One of the biggest - and best-preserved - of the Maya city-states, Palenque is full of temples, palaces and marketplaces that researchers believe date back to around 600-900 AD. Nobody is sure why the Maya civilisation was destroyed, and its great cities such as Palenque abandoned, but theories include war and famine. The decayed site has been restored and is a popular tourist spot.

In pictures: Weird and wonderful sites

Amazing alien landscapes

Derinkuyu, Turkey

The largest and deepest of 200 underground cities in the Cappadocia region, this eerie location was home to approximately 20,000 people (plus livestock, a church, school and kitchen). The inhabitants dug tunnels and rooms beneath their homes in the soft volcanic rock.

The city reportedly grew to 85 metres and 11 levels deep. It is believed to date back to the early Byzantine Empire, as early as the 7th-8th centuries.

People fled to the area to find safety from anti-Christian Romans, bandits, and later on, anti-Christian Muslims. Huge rocks were rolled across the entrances, with air shafts letting fresh air in. It was sealed up at some point after the 10th century but reopened in 1969.

Catalhoyuk, Turkey

Believed to be one of the world's earliest urban settlements, thousands of people called Catalhoyuk home in 7400BC - 6200BC. And they did things quite differently to us.

In a unique design, the city was built like a honeycomb, with walls shared between different houses and doors cut in the roof so people had to climb on top to get inside. What's more, the dead were buried in the floor of their homes.

It's not clear what happened to the inhabitants of this ancient city.

Pompeii, Italy

One of the most famous ancient sites, Pompeii is a city frozen in time. Believed to have been founded in the 6th or 7th century BC, it was almost obliterated when Mt. Vesuvius erupted in 79AD. Estimates vary, but more than 1000 people were killed instantly, and the town was buried and forgotten. That is, until it was rediscovered 1800 years later.

It's a fascinating place to visit as it has been perfectly preserved. There are many historical details alien to us such as bizarre art and graffiti, decorative penis statues and weird living arrangements. A resort town, it also has public baths, a brothel, amphitheatre and a hotel. Approximately 2.5 million people visit Pompeii every year.

Machu Picchu, Peru

It's the destination at the top of many travellers' bucket lists. The spectacular 'lost city' of Machu Picchu in Cusco was discovered in 1911 and is one of the most famous sites created by the Inca Empire.

In a remarkable feat of 15th-century construction, the Incas flattened the top of the 2430 metre high mountain to accommodate 140 structures including temples and houses. The city was divided into areas for royalty and the lower classes.

And it's not the jawdropping architecture that's the most puzzling part of Machu Picchu. How they ran a vast empire in an isolated area of Peru without building any marketplaces is quite puzzling, and dramatically different to most other old cities, where market squares were key. Why did they have no recognisable economy, and how did they prosper without it?

Cahokia, US

It's believed between 10,000 to 40,000 people called Cahokia home for hundreds of years. Surrounded by huge earthen pyramids and wooden structures, it's estimated the civilisation was at its height between 600-1400 AD. The inhabitants of Cahokia built huge markets and had sophisticated agricultural practices. More than 120 mounds were built here. It's not known why the city was abandoned in the 1200s, but the depletion of resources and disease may have played a part.

Great Zimbabwe

The giant, walled and wealthy city of Great Zimbabwe was home to around 30,000 people at its peak in 1200-1450. An important trade centre, it was rich in gold from local mines. The technologically advanced city features a huge enclosing wall some 20 metres high and was believed to have served as a royal palace for the Zimbabwean monarch. Famines may have contributed to its mysterious demise.