An ancient Mayan pyramid that has fascinated explorers since its discovery by Spanish conquistadors in the 1500s continues to reveal its secrets.

Experts have discovered a secret passageway that they believe leads to an underground water-filled cave, or cenote, at the Temple of Kukulkan in Mexico's Chichen Itza.

There are stories of the Mayans sacrificing people into cenotes with a previous expedition finding human remains in the watery sinkhole.

Researchers hope the blocked-off secret passageway will reveal the 'sacred geography' of the site along with new details about the Mayans ancient beliefs.

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The ancient Mayan temple of Kukulkan continues to reveal its secrets a millennia after it was constructed. Researchers used ground penetrating lidar (foreground) to reveal an underground cavern beneath the city's central pyramid (background)

THE UNDERGROUND CENOTES Kukulkan is a Mayan snake deity, or a feathered serpent, who grew up inside a cave before emerging in an earthquake. The central cenote is thought to be connected to other sinkhole lakes that surround the pyramid to the north, west and east, by underground rivers. These surround cenotes could represent the four points of the compass. The river at the centre might represent the centre of the Maya's universe, which they thought of as a tree with roots reaching below ground. There are stories of the Mayans sacrificing people into cenotes with a previous expedition finding human remains in the watery sinkhole. Advertisement

Researchers from the Great Mayan Aquifer Project, led by underwater archaeologist Guillermo de Anda, made the discovery using advanced imaging techniques.

They used ground penetrating lidar, a form of radar, to send electromagnetic signals through the walls and other architectural elements of the pyramid and its surroundings, to map the underground of the temple's main structures.

So far they they know the tunnel exists, but they are now hoping to find an entrance to the passage and hope to explore it physically.

The passage was found through a smaller burial chamber, known as the Ossuary, although it is currently blocked off.

Speaking to El Universal, Dr de Anda said: 'Through the Ossuary, we can enter the cave beneath the structure and there we found a blocked passageway, probably closed off by the ancient Mayans themselves.

Lidar, a form of radar, was used to send electromagnetic signals through the walls and other architectural elements of the pyramid and its surroundings, to map the underground of the temple's main structures

'We will enter again and this time we will try to open it to see if the passageway leads us to the entrance of the cenote beneath the pyramid.'

Researchers discovered an enormous sinkhole beneath the 1,000-year-old Temple in 2015.

They fear the body of water in the cenote, which has a river running through it, may eventually cause the entire pyramid to collapse if its roof gives way.

It is believed the Mayans may have built the Temple of Kukulkan around 900 to 1,100 years ago on top of the cavern as part of their religious beliefs.

The team has been exploring the underground labyrinth inside the temple for around six months. This image shows a tunnel that reaches down inside the Kukulcan pyramid at Chichen Itza

The passage was found through a smaller burial chamber, known as the Ossuary, although it is currently blocked off. This image shows one of the researchers shining a light on a monument within the temple

THE TEMPLE OF KUKULCAN: BUILT IN THREE STAGES The famous Temple of Kukulcan, also referred to as the Kukulcan pyramid or El Castillo, dominates the archaeological site of Chichen Itza in the Yucatan state of Mexico. The step-pyramid was built by the Mayan civilisation and served as a temple to the feathered serpent god Kukulcan. Archaeologists have known that a smaller pyramid is encapsulated underneath the visible temple since the 1930s. The step-pyramid was built by the Mayan civilisation and served as a temple to the feathered serpent god Kukulcan. A discovery in 2016 revealed an even smaller structure inside the outer two layers - with experts noting the structure's similarity to a set of nesting dolls. The multi-layered pyramid is thought to have been built in three stages. Kukulkan is a Mayan snake deity, or a feathered serpent, who grew up inside a cave before emerging in an earthquake. The found structure is thought to have been built between 500 and 800 AD, while the middle layer is thought to have been constructed between 800 and 1000 AD. The largest structure is thought to have been created between 1050 and 1300 AD. Advertisement

Kukulkan is a Mayan snake deity, or a feathered serpent, who grew up inside a cave before emerging in an earthquake.

The cenote is thought to be connected to other sinkhole lakes that surround the pyramid to the north, west and east, by underground rivers.

The sinkhole beneath the temple is around 82 feet (25 metres) by 114 feet (35 metres) and up to 65 feet deep (20 metres).

Researchers from the Great Mayan Aquifer Project, led by underwater archaeologist Guillermo de Anda, shown descending into the depths of the temple, made the discovery

The team has been exploring the underground labyrinth inside the temple for around six months. This image shows Dr de Anda crawling through a narrow space beneath the surface

There is a layer of limestone about 16ft (5 metres) thick at the top of the cenote, which the pyramid is currently sitting on.

The cenotes that surround the pyramid could represent the four points of the compass, researchers claim.

The river at the centre might represent the centre of the Maya's universe, which they thought of as a tree with roots reaching below ground.

This isn't the only mystery inside the temple.

In 2016, archaeologists uncovered a second structure within the famous pyramid

WHAT CAUSED THE COLLAPSE OF THE MAYAN CIVILISATION For hundreds of years the Mayans dominated large parts of the Americas until, mysteriously in the 8th and 9th century AD, a large chunk of the Mayan civilisation collapsed. The reason for this collapse has been hotly debated, but now scientists say they might have an answer - an intense drought that lasted a century. Studies of sediments in the Great Blue Hole in Belize suggest a lack of rains caused the disintegration of the Mayan civilisation, and a second dry spell forced them to relocate elsewhere. The theory that a drought led to a decline of the Mayan Classic Period is not entirely new, but the new study co-authored by Dr André Droxler from Rice University in Texas provides fresh evidence for the claims. The Maya who built Chichen Itza came to dominate the Yucatan Peninsula in southeast Mexico, shown above, for hundreds of years before dissappearing mysteriously in the 8th and 9th century AD Dozens of theories have attempted to explain the Classic Maya Collapse, from epidemic diseases to foreign invasion. With his team Dr Droxler found that from 800 to 1000 AD, no more than two tropical cyclones occurred every two decades, when usually there were up to six. This suggests major droughts occurred in these years, possibly leading to famines and unrest among the Mayan people. And they also found that a second drought hit from 1000 to 1100 AD, corresponding to the time that the Mayan city of Chichén Itzá collapsed. Researchers say a climate reversal and drying trend between 660 and 1000 AD triggered political competition, increased warfare, overall sociopolitical instability, and finally, political collapse - known as the Classic Maya Collapse. This was followed by an extended drought between AD 1020 and 1100 that likely corresponded with crop failures, death, famine, migration and, ultimately, the collapse of the Maya population. Advertisement

Researchers discovered the new structure using a non-invasive technique in order to avoid damaging the pyramid.

A process called tri-dimensional electric resistivity tomography', or 'ERT-3D' was used.

Reserachers hope to create a 3D map that confirms the 'sacred geography' which is thought to have guided the Mayans in the design of the city. This image shows another nearby pyramid

This involved scanning the pyramid using a series of electrodes placed around the site.

These sent electrical currents that were used to measure resistance in electrical current flow in order to digitally map any existing structures.

Experts have long known that a smaller pyramid is encapsulated underneath the visible temple.

The new study detected an even smaller structure inside the outer two layers.