On January 10 and 11, 2026, scientists from the University of Life Sciences in Wrocław conducted a bat count in the Międzyrzecki Rejon Umocniony, a vast underground concrete gallery system located west of Poland. This area, part of a military structure built by the Reich between 1934 and 1944, stretches for about 32 kilometers underground and maintains a constant temperature of 10 °C and complete darkness. These conditions are ideal for nine species of bats, and the count of 36,578 bats marks it as the largest hibernation site in Poland. However, the number is a preliminary estimate and requires further verification. The Ostwall, or the fortified front of the Oder-Warta arc, is a defensive line that spans over 100 kilometers, stretching from the Oder River in the south to the Warta River in the north. The central part, near Międzyrzecz in the Lubusz Voivodeship, includes the 32-kilometer underground system, which once connected surface bunkers and was used by a narrow-gauge railway. After World War II, the system was abandoned and stripped of its equipment, leaving behind concrete corridors and rails. The Natura 2000 site "Nietoperek" covers 7,377 hectares above this underground system. Researchers have studied bats in the area for over 40 years. The stable conditions of the galleries provide ideal hibernation environments. The greater mouse-eared bat, the most common species, prefers temperatures between 7 and 10 °C. Nine species regularly hibernate at Nietoperek, and up to twelve have been recorded. The greater mouse-eared bat, Daubenton's bat, and Natterer's bat are among the most common. Surface bunkers, which are cooler, are used by species like the European barbastelle and the brown long-eared bat. Some of the bats tagged during the count travel more than 250 kilometers to reach the site. These tags have helped researchers trace the bats' origins, which cover an area of at least 17,000 square kilometers, including parts of Brandenburg, Mecklemburg-West Pomerania, Saxony-Anhalt, and western Poland. This area is nearly twice the size of Corsica. To protect the hibernating bats, the galleries are closed to tourists from October 15 to April 15. Winter counts have been conducted since 1999 as part of an international program coordinated by the University of Wrocław. To minimize disturbance, all activities must be completed in a single day, from sunrise to sunset. With 32 kilometers of corridors, this requires at least 50 people. For the 25th edition in January 2026, researchers from countries including Ireland, the United Kingdom, Germany, Portugal, Spain, and even Honduras traveled to the site. The 2026 count showed an 8 percent decline compared to the 39,768 bats recorded in 2025, but such fluctuations are not uncommon. The final results, expected about two weeks after the count, will include data on bats outside the main network, such as those in surface bunkers. Researchers have noted that three species preferring colder conditions—barbastelle, brown long-eared bat, and Daubenton's bat—are declining due to rising winter temperatures. In contrast, the greater mouse-eared bat has increased in numbers, benefiting from the slight warming. Researchers monitor these changes closely, as bats are highly sensitive to climate variations, producing only one offspring per year. According to researcher Tomasz Kokurewicz, a single bat can consume up to 500 mosquitoes and 1,000 flies in a single night, highlighting their vital role in controlling insect populations.