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Origins -

Back in the 1930s when the Nazis decided they wanted to strengthen their eastern border, they set about building an underground system of fortifications.

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Origins -

The system was built in an area of Germany around 120 km east of Berlin. Nowadays that area is part of western Poland, near to a town called Międzyrzecz.

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System -

The system, which came to be known as the Ostwall, consists of a number of underground bunkers. The bunkers are linked by tunnels and corridors with a total length of 32 km.

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System -

There are also 50 above-ground firing points and interesting-looking sets of “dragon’s teeth,” which were originally built for the purpose of deterring tanks.

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Changing purpose -

Work on the site began in 1936, but was halted by 1938 due to budget restrictions. After Germany invaded Poland in 1939, the tunnels became redundant as border protection.

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Changing purpose -

When Germany was defeated in 1945, the system was completely abandoned and over time it has taken on a different and definitely unexpected role.

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Bat hibernation site -

Nowadays the former Ostwall fortification serves as the largest bat hibernation site in northern Europe.

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Bat hibernation site -

The man-made tunnels at Ostwall are particularly important as a bat hibernation site because there are no naturally-occurring sites in the area.

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Yearly migration -

Every year some 35,000 bats of 12 different species retire to the tunnels at Ostwall to bed down and see out the bitterly cold winter months.

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Yearly migration -

The reason for the bats’ yearly hibernation is that for several months during the winter the insects that they usually feed on are nowhere to be found.

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Hibernation -

Going into hibernation allows the body processes to slow down, which in turn enables the bats to survive on their existing energy reserves.

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Requirements for hibernation -

The three main requirements for a bat hibernation site are consistently cold temperatures, high humidity, and safety, with safety being the most important.

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Optimal conditions -

These ex-military tunnels make for the perfect site because they are very safe–with three levels they go deep underground and, of course, are no longer used.

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Annual bat census -

Every year scientists and volunteers from across Europe grab their winter gear and head to the conservation site to carry out a winter bat census.

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Annual bat census -

The winter bat census is quite an extensive exercise given the size of the site and the not inconsiderable number of bats in hibernation there.

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Challenges -

The researchers conducting the census must be careful to keep disturbance to a minimum, so as to avoid awakening the bats and causing them to use up their precious energy reserves.

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Challenges -

Although some bats are in clusters on the walls of the tunnels, others are in alcoves and crevices and they can therefore be very difficult to count.

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Challenges -

This difficulty is compounded by the fact that the bats are often sitting on top of one another, meaning that many are hidden and missed at first sight.

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Mitigation -

Risks of miscounting are mitigated by having several people count the same cluster and coming to an agreement as to the total number of bats in the cluster.

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Hazards -

There are also a considerable number of hazards in the tunnels of which researchers must be acutely aware, such as unexpected holes in the ground.

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Importance of the census -

The bat census is not an easy exercise, but it is a very important one, since it is this process that allows scientists to see whether there are any problems affecting the bats in the area.

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Importance of the census -

As a rule, the status of wellbeing of the bats in this area is also a good indicator of how well the environment and surrounding habitat is doing in general.

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Importance of the census -

To quote Henry Schofield of the Vincent Wildlife Trust, "If the bats are doing well, it means that the environment is good. If the bats are doing poorly, it means that there is something wrong."

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Tourist site -

During the winter, the tunnels are fully closed to tourists so as to allow the bats to hibernate in peace. At other times of the year, the site is open to visitors.

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Tourist site -

While many visitors are tempted to go off and explore the tunnels on their own, there is in fact a museum at the site that will take visitors on an informative tour.

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Tourist site -

The Fortification Museum also has its own (relatively small) exhibition, which you may want to visit to learn about the history of the Ostwall before embarking on a tour of the tunnels.

(BBC) (Times of Israel) (Landmark Scout)

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