An awe-inspiring Nazi defence line in western Poland has taken on a new role in peacetime as home to tens of thousands of bats in what is Europe's largest artificial roost.

The 37,000 winged mammals sleep elbow-to-elbow in the well-sheltered tunnels of the Ostwall fortification, a largely forgotten war site near the town of Miedzyrzecz, not far from the German border.

Adolf Hitler had it built on the eve of World War II in what was then German land to protect the Third Reich from a hypothetical attack by Poland or the Soviets, though it proved fairly useless.

Today, it doubles as a tourist site and massive bat reserve, and since 2011 has been home to what is likely the world's only combined fortification and bat museum.

"Europe's largest bat hibernation site is in a Romanian cave. But here we have the largest man-made one," said Jan Cichocki, zoologist at the nearby University of Zielona Gora.

"The bats really have it good here. They have nothing to fear," he told AFP.

The defence line -- which was also known as Festungsfront im Oder-Warthe-Bogen -- spans 60-plus kilometres (40 miles) and includes more than 100 bunkers, obstacles and other fortifications, according to museum director Leszek Lisiecki.

The whole could accommodate 24,000 soldiers, though staffing was never that high and by the end of the war only a few hundred men remained.

A middle section features several large bunkers that are connected by 33 kilometres of tunnels hidden up to 40 metres (130 feet) underground -- a fine bedroom for a bat.

"The area is perfect because of a steady temperature and humidity," said Cichocki, who is carrying out a bat census and observation project in the pitch-black corridors.

The temperature hovers between seven and 10 degrees Celsius (45 and 50 degrees Fahrenheit), while the humidity reaches 90 percent in certain places -- both ideal conditions for bats wanting to catch some Zs.

Cichocki says their number varies from year to year, but for the past two years there have been around 37,000 from a dozen or so species to hit the sack here from October to April.

"Spring came early this year, so some of the species have already flown up to the surface to look for food and form breeding colonies," he said of the flying mammals.

But some of the greater mouse-eared bats -- scientifically known as Myotis myotis -- are still roosting in the tunnels, huddled together with their heads down in clusters of several thousand.

- Let sleeping bats lie -

In snooze mode, a bat's body temperature falls to a low just above that of its surroundings. Vital functions also slow, with the heart beating just a couple of times a minute.

From time to time one of the bats will stretch in its sleep or yawn, showing off a mouthful of pointy fangs.

Rousing the nocturnal creature prematurely is inadvisable: the tremendous energy required for it to restore its functions to a wakeful state may cause it to drop dead.

That is why many of the Ostwall tunnels are closed off during the winter, made inaccessible to humans by padlocked gates.

"It's a reserve. You need a permit to enter. It's our way of protecting the bats against vandals and tourists," Cichocki said.

The fortification's awe-inspiring size, advanced design, and attention to troop comfort have attracted ever-increasing numbers of visitors in recent years.

"Each soldier had his own bed. There were bathrooms, medical stations, all of life's basic necessities. The kitchen was equipped in such a way that it would pass a health inspection even today," said Lisiecki, the museum director.

Like France's Maginot Line, Ostwall had but a bit part in the war. Overtaken by evolving military designs and lacking the necessary staff, it quickly fell into the hands of the Red Army.

After the war it served as a firing range for the Polish army, as well as a mecca for looters and adventurers.

Communist officials planned to build a nuclear waste repository there in the 1980s but abandoned the idea when local residents protested.

It was around then that zoologists began noticing the bats.