The Osowka Underground City, located in the picturesque village of Sierpnica, is one of the best-preserved and most mysterious elements of the Riese complex. This underground fortress, hollowed out in the slope of the Osowka mountain, is the second largest site of the Riese project, with an impressive network of corridors with a total length of nearly 9 kilometers and an area of more than 26,000 square meters. The volume of all hollowed out rooms is estimated at about 100,000 cubic meters, which gives an idea of the scale of the project.

The facility consists of several parts connected by a system of corridors. The main tourist route leads through concrete structures that were probably intended to serve as bunkers or production halls. Visitors can admire the massive reinforced concrete supports, massive walls and an intricate drainage system that still functions today, draining water from inside the mountain. Particularly impressive are the huge halls several meters high, whose purpose is still the subject of speculation among historians.

Osowka offers several sightseeing routes with varying degrees of difficulty. The extreme route, designed for lovers of stronger sensations, leads through partially flooded corridors, where participants equipped with the appropriate equipment (wellingtons, overalls and helmets with flashlights) can feel like real explorers. A historical route is available for less skilled tourists, which also allows them to learn the secrets of the place, but in more comfortable conditions.

The history of Osowka's construction is as dark as its corridors. Prisoners from the nearby Gross-Rosen concentration camp and its branches were used for tunneling work. They worked in inhumane conditions, often without adequate food, clothing or tools. Thousands of them paid for this work with their lives - some died of exhaustion, some were murdered by guards. During the tour of the complex, the guides tell about the tragic fate of these people, which gives the visit to this place not only an adventure, but also an educational and reflective dimension.

It is worth noting the technical details of the building - traces of formwork on the concrete walls, the ventilation system, the remains of the electrical system. All this testifies to the technological sophistication of the project and the huge resources the Nazis allocated to its implementation. Particularly interesting are traces of the tracks of the narrow-gauge railroad, which was used to transport excavated material and building materials inside the complex.

The Riese (German for 'Giant') project is Nazi Germany's largest World War II mining and construction project. Despite the years that have passed, it is still not entirely clear what the true purpose of these huge underground complexes was. The most popular theory is that it was to be Adolf Hitler's secret headquarters, similar to the Wolf's Lair, but much better hidden and protected. Other hypotheses suggest that the underground of the Owl Mountains was to have been home to weapons factories, including perhaps even secret laboratories working on nuclear weapons.

Historians point to several facts that may support the theory of Hitler's quarters. First, at the same time, intensive renovation work was underway on the nearby Książ Castle, which was likely to be part of the same complex. Second, the scale of the project and the resources involved suggest that the project was a top priority for the Nazi leadership. Finally, surviving documents indicate that an exclusion zone called 'Schlesierland' was planned in the vicinity of the Owl Mountains, which also fits the theory of the Führer's quarters.

On the other hand, supporters of the weapons production theory point to the proximity of the uranium mine in Kowary and the testimony of some witnesses who claimed to have seen shipments of mysterious equipment and materials being brought to the complex. Some researchers even suggest that the Owl Mountains may have been the site of experiments with technologies ahead of their time, including the legendary 'Bell' - a mysterious device that was supposed to be a revolutionary weapon or even a machine for manipulating space-time.

One of the most intriguing legends associated with the Riese complex is the story of the so-called 'Golden Train.' According to the story, in the last days of the war, the Germans allegedly hid a train filled with gold, artwork and other valuable items looted from all over Europe in one of the tunnels. Although in 2015 two treasure hunters announced that they had located this train near Walbrzych, subsequent research has not confirmed its existence. Nevertheless, the legend continues to attract adventurers and treasure hunters to the region.

Another fascinating mystery is the fate of the lost documents of the Riese project. Most of the plans and technical documentation disappeared in the chaos of the end of the war. Some may have been destroyed by the Germans themselves, while others were seized by the Red Army and taken to the USSR. This gap in documentation means that many aspects of the complex remain unexplained, which only fuels speculation and conspiracy theories.

Also worth mentioning are the mysterious deaths and disappearances of people who tried to investigate the Riese complex during the communist era. Some enthusiasts suggest that this may have been related to secret operations of the secret services, who wanted to hide the truth about what was really in the underground of the Owl Mountains. These stories, while difficult to verify, add another layer of mystery to an already fascinating place.

Exploring the underground complexes of the Owl Mountains requires adequate preparation, both in terms of equipment and knowledge. The Osowka Underground City offers organized guided tours, which is by far the best option for first-time visitors. Various routes with varying degrees of difficulty are available, so everyone can choose a variant that suits their abilities and expectations.

The historical route is the easiest and most accessible - it can be taken in ordinary shoes and clothing. It leads through the main secured corridors and halls of the complex, and a guide tells the history of the construction and the probable purpose of each room. This route takes about an hour and is accessible all year round.

For more demanding tourists, there is an extreme route, which leads through less accessible parts of the complex, including partially flooded corridors. To complete it, you will need wellingtons or waterproof shoes, warm clothing (the temperature in the underground remains at about 5-7 degrees Celsius all year round) and a helmet with a flashlight, which can be rented on site. This tour gives you the opportunity to see places inaccessible to ordinary tourists and feel the atmosphere of real underground exploration.

The most advanced option is the expedition route, designed for people in good physical condition and without fear of darkness or cramped spaces. It takes you through the most remote and hard-to-reach parts of the complex, where you sometimes have to squeeze through narrow passages or wade through water. This route gives the most authentic urbex experience, but requires proper preparation and equipment.

In addition to Osowka, there are several other sites of the Riese complex available to visitors in the Owl Mountains. The largest of these is Wlodarz, a partially flooded complex of about 9,000 square meters. A tour of this site includes a boat ride through the flooded corridors, which is an added attraction. Another interesting site is Rzeczka, a smaller complex, but also offering the opportunity to see impressive underground structures.

For those interested in World War II history, it is worth combining a tour of the underground with a visit to the Gross-Rosen Museum in Rogoznica, where you can learn more about the fate of the prisoners who built the Riese complex. The museum is located on the grounds of a former concentration camp and is an important memorial to the victims of Nazi terror.

It is worth remembering that exploring the underground outside the designated tourist routes is dangerous and in most cases illegal. Many of the unguided adits are at risk of collapse, flooding or contain other dangers, such as unmarked shafts or unstable structures. In addition, some sites harbor protected bat species and entering them can disturb their natural habitat.

The best time to visit the underground complexes of the Owl Mountains is late spring, summer or early autumn, when temperatures outside are higher, making it more comfortable to stay in the cool underground. However, it is worth remembering that the sites are very popular during the summer season, so to avoid crowds, it is best to plan a visit on weekdays or early in the morning.