Today’s Wonder of the Day was inspired by Jameson. Jameson Wonders, “Where are the catacombs?” Thanks for WONDERing with us, Jameson!

If you live in a large city like New York City or Los Angeles, the idea of tunnels underground might not seem very strange. After all, these two cities have plenty of subway tunnels that transport people underneath the streets of the city at all hours of the day.

But what about deep, dark passageways that hide secrets centuries old? If you live in Rome, Paris, Odessa, New York City, London, Malta, or Alexandria, your city features a wide variety of these labyrinthine passageways, known as catacombs.

Some of the first catacombs ever constructed were built in Rome in the 1st century. At that time, bodies could not be buried within the city walls, so catacombs were built as underground tombs by early Jewish and Christian communities.

Today, the most famous catacombs exist under the city of Paris. The tunnels that stretch for over 185 miles beneath the city are leftover passageways from the 13th century, when limestone was quarried from underground to build Paris into the cultural hub it is today.

They remained largely unused until the late 17th century when they became the solution to a pressing public health problem. By 1780, the cemeteries of Paris were overflowing with corpses, causing foul odors to permeate local neighborhoods and creating the perfect conditions for the spread of disease.

Beginning in 1786, cemeteries were slowly emptied, as remains were transferred into the tunnels below the city. Ossuaries were created to store the bones of more than six million Parisians who had formerly been buried in the city's cemeteries.

The catacombs of Paris served as more than just an underground tomb, though. During World War II, the underground tunnels were used by both French Resistance fighters and German soldiers.

Visitors to Paris today can now take guided tours of more than a mile of the catacombs. The vast majority of the catacombs, however, remains off-limits and is illegal to explore.

If the catacombs of Paris sound like a massive labyrinth, you won't believe the network of tunnels that exist beneath Odessa, Ukraine. Considered the world's longest network of catacombs, the tunnels under Odessa stretch nearly 1,500 miles!

Like the Paris catacombs, the tunnels under Odessa were carved out in the 1830s when limestone was mined. Also like the Paris catacombs, the Odessa catacombs were used during World War II by soldiers and can now be toured by the public as part of a museum.

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