Zbiornik Wody Stara Orunia: An Underground Industrial Reservoir and Protected Nature Reserve

The history of Zbiornik Wody Stara Orunia begins in the mid-19th century, when Gdansk undertook a major modernization of its water and sewage systems under the leadership of mayor Leopold von Winter. The Prussian engineer Eduard Friedrich Wiebe designed both the overall water supply concept and the specific structure of the reservoir as part of this program. Construction was completed in 1869, and the reservoir entered service as a gravity-fed drinking water cistern serving the urban network.

Water reached the reservoir through a cast-iron pipeline from a drainage intake at Pręgowo, located above the city. From the reservoir, water descended under gravity into the lower parts of Gdansk, allowing the system to smooth out daily fluctuations in demand. Because the reservoir was built underground on a hillside rather than as an elevated tower, it functioned as a quieter, less conspicuous counterpart to a traditional water tower.

The reservoir belonged to the low-pressure category of Gdansk's network of several underground cisterns, owing to its modest elevation of approximately 40 meters above sea level. By the late 20th century this became a limitation, and the structure was taken out of active service in 1978. Although the reservoir itself was retired, the original Pręgowo water intake has continued in use, now feeding newer reservoirs near the historic site.

Between 2016 and 2018 the abandoned reservoir was revitalized for public access and heritage preservation. Works included uncovering and conserving the brick vault ceilings and replacing the original earth covering with lighter expanded clay aggregate to reduce structural load and improve insulation. In 2019 the site was entered into the municipal heritage register, and it has since been incorporated into the Gdansk Water Trail, a publicly accessible route connecting surviving historic water infrastructure across the city. Since 2021 the site has also been designated as a Natura 2000 special area of conservation for bats, marking a new chapter in which the historic reservoir is valued simultaneously as cultural heritage and as protected wildlife habitat.

The natural environment of the Orunia reservoir is unusual because the most important ecological feature lies underground, inside the brick structure itself. The reservoir's enclosed chambers maintain a particularly stable microclimate: constant darkness, limited air exchange, mild and even temperatures, and very low disturbance from human activity once the hibernation season begins. These conditions closely resemble traditional cave environments, which are scarce in the lowlands of Pomerania, and they form the basis for the site's ecological significance.

The broader landscape around the reservoir belongs to the Dolina Potoku Oruńskiego protected landscape complex, a zone that protects the valley of the Orunia stream along with its associated habitats. This mosaic includes fragments of woodland, wet meadows, and small watercourses that connect the reservoir area with the nearby Radunia Canal. Together, these features support a network of habitats used by bats both for roosting and, in warmer seasons, for commuting and feeding.

While the reservoir itself is a built environment, its role in the local ecological system is essentially that of an artificial cave within an otherwise lowland urban matrix. For bats that require cool, quiet, and frost-free winter quarters, the difference between a natural limestone cave and a brick vault maintained at roughly stable temperatures is far less important than the absence of disturbance. The site's Natura 2000 status reflects precisely this recognition that even small, very localized shelters can be critical on a regional scale.

Bats are the dominant wildlife focus at Zbiornik Wody Stara Orunia, and the reservoir is now classified as a Natura 2000 special area of conservation specifically to protect its hibernating bat community. The primary species protected is the greater mouse-eared bat, whose winter roost in the reservoir is the formal object of conservation for the site.

In addition to the greater mouse-eared bat, the reservoir is regularly used by other bat species during the hibernation period. Surveys have recorded the presence of the whiskered bat and Natterer's bat, with occasional observation of the long-eared bat and the brown long-eared bat. Since the early years of monitoring, winter counts have varied considerably from year to year, with totals ranging from roughly 100 to about 800 individuals, reflecting natural fluctuations in bat populations and local conditions.

Visits to the reservoir are timed to avoid the hibernation period, so as not to disturb the bats. Because the underground chambers hold still water and quiet air throughout the cold season, and because access is tightly regulated, the reservoir has become one of the most reliable urban bat wintering sites in the Gdansk area. For visitors and local conservation organizations alike, the site illustrates how a piece of decommissioned industrial infrastructure can acquire significant ecological value once managed for wildlife.

Conservation at Zbiornik Wody Stara Orunia operates on two interlocking levels: the protection of an industrial heritage structure and the protection of an important bat hibernation site. Since 2021, the underground reservoir has been recognized as the Natura 2000 site 'Zbiornik na Oruni,' a special area of conservation focused on the greater mouse-eared bat and its winter roost within the brick-vaulted chambers.

The most direct conservation measure is the strict limitation of human access during the bat hibernation season, when disturbance could be fatal to roosting animals. Public visits are only possible outside this period, and only with a guide, which limits the number of people entering the chamber at any one time and ensures that bat-friendly conditions are maintained. The 2016 to 2018 revitalization was carried out with conservation in mind: excavating and restoring the vaults improved their structural stability, while replacing the heavy earth cover above with lighter expanded clay aggregate reduced the structural load and improved thermal insulation, helping to stabilize conditions inside.

The reservoir is also part of the broader Dolina Potoku Oruńskiego protected landscape complex, which connects the site to surrounding natural areas. Recognition as a Natura 2000 area and inclusion in Gdansk's municipal heritage register in 2019 have together given the site a formal, long-term protective framework. In effect, the conservation story at the Orunia reservoir is one of reusing decommissioned infrastructure for biodiversity rather than removing or replacing it.

Zbiornik Wody Stara Orunia is closely tied to the identity of the Orunia district, one of Gdansk's older neighborhoods, with a long history of small-scale industry, market gardening, and freight along the Radunia Canal. The Orunia Park nearby, the nearby hills along the canal valley, and the legacy of local brickyards at Kolibki together shaped both the labor and the materials that made the reservoir possible.

The site also carries an explicitly educational and touristic cultural role. Since its revitalization, the reservoir has been integrated into the Gdańsk Water Trail, a route that opens up surviving pieces of the city's historic water supply for guided visits. Adjacent viewpoints, including Viewpoint XII 'Jerzy,' named for the historian Jerzy Samp, emphasize the reservoir's place in a wider narrative of local heritage, scholarship, and community memory. For residents and visitors alike, the reservoir represents a tangible link between 19th-century public health engineering and contemporary urban identity in Gdansk.

Zbiornik Wody Stara Orunia stands out as one of the rare underground water reservoirs from the Prussian era of Gdansk's history that remains physically accessible. Its defining features include the eleven parallel brick naves, the 19th-century cast-iron pipeline system that once brought water from Pręgowo, and the unusually calm, cathedral-like interior formed by brick arches over still water.

Equally notable is the site's layered identity. It functions simultaneously as a piece of preserved industrial heritage, a stop on the Gdańsk Water Trail, and a Natura 2000 protected area hosting one of the region's most important bat hibernation roosts. This combination of architectural, educational, and ecological value is uncommon even among European historic waterworks and gives the Orunia reservoir a distinctive cultural and natural profile.

The most suitable period to visit the reservoir is outside the bat hibernation season, when guided tours are offered and disturbance to wildlife is avoided. Visiting during late spring, summer, or early autumn allows exploration of the underground chambers when bat activity is reduced and the surrounding Orunia Park and Dobra Potoku Oruńskiego valley are at their most attractive.

In winter, the site itself is closed to visitors to protect hibernating bats, but the surrounding landscape still offers viewpoints and walks. Weather in Gdansk is generally mildest and driest from late spring through early autumn, which is also when the wider Radunia Canal valley and adjacent parks are most accessible. Because tours are available only with a guide during a limited season, visitors should plan a trip within the open months rather than attempting a winter visit.