(CNN) — Known as the Pearl of the Black Sea, Odessa is one of the most captivating destinations in Ukraine.

But while its magnificent architecture and beautiful beaches are relatively well known, the most enthralling destination here is actually located way beneath the city.

Deep in the neighborhood of Moldavanka, inside an unassuming garage located off a busy road, lies a special gateway to one of the world's largest urban labyrinths -- the Odessa Catacombs.

On entering, visitors are handed a helmet and a flashlight before descending around 25 meters (82 feet) down into the darkness.

Buried city

Unlike the catacombs of Paris and Rome, the Odessa catacombs were never used to bury the dead. Pavlo Fedykovych

With more than 2,000 kilometers (1,243 miles) of tunnels, this underground city is so vast and complicated -- there are at least 1,000 known entrances -- that tourists are advised not to enter without a qualified guide.

Its size seems particularly staggering when compared to the catacombs of Rome and Paris, which span 300 and 500 kilometers respectively.

But unlike those in the French and Italian cities, these underground passages were never used to bury the dead.

Now a chaotic system of unconnected caves and abandoned quarries or tunnels spread over three levels, the story behind their existence begins around the time Odessa was founded at the end of the 18th century.

Coquina, a sedimentary rock, largely contributed to the city's rapid growth as it was used to construct the majority of buildings in Odessa at the time.

The principle was simple: if you wanted to build yourself a palace, the easiest way to find the material was to set up a quarry and mine as much coquina as you needed.

When the city experienced a huge commerce boom between 1819 and 1859, many palaces were erected, which meant countless quarries were dug underground.

This process was pretty much unregulated, going some way to explain how large and disordered the area is now.

The catacombs underneath the Moldavanka neighborhood form part of the Secrets of Underground Odessa museum

Bomb shelter

An anti-nuclear Cold War bunker is one of the many eerie sights along the network of tunnels. Pavlo Fedykovych

With only a weak light trail to follow, visitors are met with chilly temperatures of around 13 Celsius (57 F) on venturing underground.

A decaying Cold War nuclear bunker which uses the catacombs' narrow pathways its foundation is one of the first sights.

It's a pretty grim scene today. The air within the shelter is stale, while the silence is deafening.

There are rusty pieces of Soviet-era equipment cages and wires inside the unit, but the showstopper is its eerie engine room, which is almost completely submerged in water.

Next up is the so-called "wild" part of the catacombs, one of the many former coquina quarries.

This section feels almost romantic when compared to the bunker, with coal paintings etched on its walls, all with inscriptions underneath.

"There are hundreds of inscriptions, different symbols and paintings in the catacombs," explains Andriy Dembitskyi, a guide for local Tudoy-Sudoy tour agency who is passionate about Odessa.

While many of the captions indicate dates, directions, and even swear words, the meaning of those below these particular paintings is still unknown.

Underground tales

Various tools used for stone mining and Soviet memorabilia on display. Pavlo Fedykovych

Exploring the Odessa catacombs can be a fascinating and sometimes grisly experience.

Dembitskyi recounts the time his colleague found a decaying body, assumed to be a World War II casualty, in unexplored tunnels on the outskirts of Odessa.

He apparently put the remains in a bag before bringing it to the police. According to Dembitskyi, the police refused to accept the corpse, arguing it was located in the jurisdiction of one of the city's World War II museums. However, it seems the museum didn't want to take responsibility for the remains either.

In Kafkaesque fashion, the guide is said to have driven around the city for miles with the dead man in his trunk, before ending up at the prosecutor's office, who he says agreed to take it.

Another story involves a special unit of the NKVD, a Soviet secret police agency known for its role in political repression and overseeing the union's prisons and labor camps.

Approximately 32 members were sent to the catacombs in 1941 to sabotage the Romanian allies of German Nazis who occupied Odessa during World War II, when Ukraine was part of the Soviet Union.

Details of their fate were sealed under state archive for years, but the documents were finally opened to the public as part of decommunization policy a few years ago, revealing that only one member of the group ever saw the light of day again.

The unit comprised of two NKVD groups, one from Odessa and one from Moscow.

Tensions between the competitive groups resulted in a chain of shootings and betrayals acerbated by the atmosphere of the catacombs.

Many of the members were executed, while others died from various illnesses.

By 1943, only the leaders of each group remained. But that same year, the head of the Odessa unit fatally shot his Moscow counterpart and spent more than nine months underground alone.

While he did eventually emerge from the ground, he was sent back in 1944 and died in a grenade explosion.

These gruesome tales are just two of many dark stories connected to this labyrinth of tunnels, which has also been linked to smugglers and outlaws.

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Signs of life

The catacombs conceal a number of small underground lakes. Pavlo Fedykovych

As many sections remain unexplored to this day, we may never know all of the secrets that are concealed in this underground maze.

But there's more than just death and darkness to find here.

Dembitskyi points to the center of a compact underground lake, where there's some slight movement.

"Recently, we've started breeding fish here as an experiment," he explains. "So far it works."

A glimmer of life in a place where very little else has survived.

Further along, there's a display of instruments used for stone mining and various Soviet memorabilia, the reconstruction of a criminal stash dating back to the Russian Empire and even an improvised underground restaurant.

As the tour reaches its end, Dembitsky explains that today's outing has only covered about three kilometers of the catacombs, less than 1% of the total area.

Dating back to the start of the 18th century, Quarry Cantacuzene museum (or Quarry Kantakuzin), which is separate to the Secrets of Underground Odessa museum, is also worth exploring.

As the oldest underground quarry in Odessa, it offers an interesting introduction into the world of the catacombs, as well as the early history of the city.

Back on ground level, bustling Odessa feels totally different.

Now it's hard to ignore the fact that there's another city underneath -- a cold maze of never-ending tunnels.

Fight for recognition

The Moldavanka neighborhood holds one of the official entrances to the catacombs. Pavlo Fedykovych

Leonid Ashtulenko, a guide who's spent more than 50 years of his life exploring the catacombs of Odessa, is currently trying to publish a book about the inscriptions inside, his magnum opus.

Ashtulenko is seeking private funding, as the city has shown little interest in the project.

He proudly shows off the numerous photos he's taken, while enthusiastically reciting tales from the city below.

Both Dembitsky and Ashtulenko form part of a local movement to make the subterranean world more accessible to visitors.

The group have been fighting, with admirable zeal, for the catacombs to be recognized as an Odessa attraction in the same league as the Odessa National Academic Theater of Opera and Ballet and Philharmonic Theatre as well as its Black Sea beaches.

And so far they're succeeding. Interest in the catacombs rises with every year, as more and more people descend underground.

And there's no denying the allure of this eerie underground area.