Another interesting part of MRU is located in Pniewo (Fischerhaus) in the Międzyrzecz District.

In the central section the bunkers are interconnected with a system of tunnels, 32 kilometres (20 mi) long and up to 40 metres (130 ft) deep.

It is also among the top 10 largest hibernation sites for bats in Europe according to the data of the EUROBATS Agreement.

12 species have been found hibernating there and one more (the northern bat) roosts in cooler stand alone bunkers not connected with the underground system.

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