This 650-meter-long submarine base was carved into the rock in 1969. Each end had an entrance, protected by massive armored gates. Designed to accommodate eight submarines, the base is now completely abandoned.

Built in the early 20th century, this spectacular reservoir is vaulted by a dome and supported by columns. It is the most beautiful underground reservoir I have ever seen.

There is a disused tunnel in the Kyiv metro, abandoned after the construction of the third metro line. It is very difficult to access, as it can only be reached through a functioning subway line.

This unfinished command bunker in central Kyiv is one of the most impressive underground sites I have ever seen. Three underground floors were planned, but only a massive tunnel with a ten-meter diameter, connecting to the Kyiv metro, was completed.

Vilnius, like most other old European cities, has its own underground areas where you can take stunning photographs.

This tunnel was used by vacationers to reach the sea from the sanatorium above. It is heavily decorated with motifs from Ukrainian fairy tales, making it one of the most photogenic places in underground Odesa.

There are many abandoned limestone quarries near Paris. Occasionally, you can find underground lakes in them, but mostly, you will see large and tall columns that somewhat resemble the Mines of Moria from The Lord of the Rings.

Slovakia is known among urban explorers for its abandoned mines of coal, metals, and even uranium. Often, these shafts are well hidden from prying eyes and can be difficult to find.

Often referred to by urban explorers as the largest underground military structure in the world, this Nazi fortification was designed to protect Germany from an eastern attack. Between 1928 and 1938, 34 bunkers were constructed, connected by a 30-kilometer underground railway. We spent three days exploring this system, sleeping and eating underground while carrying all our supplies in backpacks.

There are many abandoned train tunnels across Europe, including in Belgium, France, Ukraine, and even Lithuania. They often provide opportunities for spectacular photos.

This quarry in the Netherlands is known for its exceptionally high ceilings, reaching heights of 15 to 20 meters.

This bunker, built for the Hungarian dictator Rákosi in the center of Budapest, was constructed in the 1950s by the communist government. It had an exit to the Budapest metro and is now one of the iconic sites of underground Budapest.

The old sewer system in Antwerp is one of the most photogenic underground places I have ever seen. Tourists are guided through parts of these tunnels, and they also sail a boat through one section filled with water. However, we managed to access it illegally without too much trouble.

Construction of this unfinished hydroelectric plant in Poland began in 1972, but less than a decade later, work was halted due to economic problems. During this time, a large network of underground tunnels was excavated, reminiscent of subway tunnels in some areas.

This train tunnel was used to access a large underground Nazi factory, where V2 rockets were produced in subterranean workshops.

The city of Odesa in Ukraine boasts not only exceptional architecture but also a fascinating network of underground labyrinths.

While many are familiar with the Odesa catacombs, I found this particular site to be the most interesting: a deep drainage tunnel that connects several separate catacombs into one large system.

Metro construction is taking place all over Europe. In most cases, it is challenging to enter these sites illegally, as work can continue around the clock. However, once inside, you can admire the expansive tunnels, which often even have lighting.

In France during the 1960s, it was popular to grow mushrooms in abandoned mines. Today, those mines are filled with the sacks used for mushroom cultivation.

Construction of this famous line of fortifications began in 1929. The line, intended to protect the border with Germany, consisted of numerous forts, each with several bunkers, all connected by underground railways. At depths of 30 to 60 meters, not only were ammunition storage facilities located, but also barracks, hospitals, rest rooms, generators, and other essential military infrastructure. Some of these forts resembled small underground cities, capable of housing thousands of people. Today, most of them are abandoned.

Can you see a small dot in the center of the photo? It is a human silhouette illuminated by a 9,000-lumen flashlight in the largest underground space I have ever seen. With dimensions of 250 by 250 meters and a height of 150 meters, it is almost impossible to photograph even with a wide-angle lens (only half of the room fits into the frame in this photo).

It was one of the largest Nazi underground factories in central Germany. According to the plans, parts for Junkers planes were to be produced in 13 kilometers of tunnels. In the 1970s, it was rediscovered by the GDR army, and an ammunition storage facility was established there. A total of 5,500 tons of ammunition, 3,500 tons of clothing and other equipment were stored in those tunnels.

This Nazi factory produced cutting-edge technology at the time, including anti-aircraft missiles like the famous Hs 117 Schmetterling. Construction of the factory began in the winter of 1944 and was carried out by concentration camp prisoners.

The construction of this underground ammunition factory began in the fall of 1944 in a former iron ore mine, at a time when Allied bombers were already wreaking havoc across Germany. According to the plans, which were never fully implemented, 25,000 square meters of underground space were to be excavated here by the hands of concentration camp prisoners by the summer of 1945.

This facility, located in a small village in Albania, is one of the most interesting places I have visited. Ammunition was produced in underground workshops filled with equipment, and you can still find full boxes of cartridges, gunpowder, and prepared bullets.

You can take a guided tour along this vast canal in the center of Vienna, or you can do what we did and explore it on your own.

This facility produced V2 engines for Nazi ballistic missiles. The engines were assembled in underground workshops, which have since been destroyed, and tested in an adjacent quarry that is now flooded.

The construction of this facility began in 1944. By the end of the year, it was already operational, an oil refinery had been established in its eastern part, and tank parts were being produced in the western section.

The history of this factory began in 1944 when a new concentration camp for 15,000 people was established here. By the end of the war, 65,000 square meters of space had been excavated in the former limestone mine for a bearing factory. Today, these tunnels are abandoned, and it is not easy to access them.

Prague is an impressive city, both above and below ground. Here, you can find many beautiful and photogenic tunnels, including both drainage and sewer systems.