On 23 December 2015 the last coal train pulled out of the Cottbus-Nord open pit and ran the short distance to the Jänschwalde power station. Nine years later to the day, the water standing in the hole that train left behind reached 62.5 metres above sea level - the target line, the mark at which engineers could finally call the flooding finished. Between those two December dates a mine turned into something German can only describe with a pun. At 18.8 square kilometres the Cottbuser Ostsee is now the largest artificial lake in Germany by surface area, and a city that spent a century looking into coal has a shoreline.
The pit opened in 1975 and gave up coal from 1981 until the end of 2015: 220 million tonnes of lignite, burned almost entirely at Jänschwalde a few kilometres to the north. Getting at it meant removing everything that lay on top, and what lay on top included people. Groß Lieskow, Klein Lieskow, Lakoma and Tranitz were excavated away completely. Dissenchen, Merzdorf and Schlichow lost parts of themselves. In all, 906 residents were resettled - a modest figure by the standards of German lignite, where whole market towns have gone under the shovel, but each of those people had an address that now corresponds to no solid ground at all. Monuments stand along the new shoreline today, one for each village that used to be out there. It is worth being precise about what happened to them. They are not lying beneath the lake. They were dug up, carried off and burned.
Flooding began on 12 April 2019 and stopped fourteen days later. The water had to come from the Spree, and the Spree did not have it to give; a dry Brandenburg spring left the river too low to divert. Filling resumed in early 2020 after better rainfall, and then it crept. By December 2020 the lake stood at 44 per cent of its target height, by March 2022 at 83 per cent, by June 2023 at 93. Those last percentage points took the longest, because a basin like this widens as it fills - every additional centimetre of depth demands more water than the centimetre below it. In March 2024 the surface sat at 60.6 metres. On 23 December it touched 62.5, and the inflow closed on roughly 126 million cubic metres of water.
The joke in the name works only in German, and only just. Ostsee is what Germans call the Baltic - the East Sea - and the new lake lies east of central Cottbus, which was all the licence anyone needed. What separates the two words is grammatical gender and nothing else: die See is a sea, der See is a lake. Cottbus therefore has an Ostsee that is emphatically not the Ostsee. There is a second name on the signage. In Lower Sorbian, the Slavic language still spoken in Lusatia, the lake is the Chóśebuski pódzajtšny jazor. Sorbian has been this region's other tongue for a thousand years, and the coalfield that produced this hole was cut straight through its heartland.
Infrastructure arrived ahead of the water, which took a certain confidence. The quay wall of the Cottbus city harbour was completed in August 2019, when the basin below it was still mostly dry ground. A sports-boat harbour with a hundred berths opened at Neuendorf in November 2022, two years before there was enough lake to fill them. Out on the surface, more than 51,000 solar modules now float across roughly sixteen hectares: a 29-megawatt array finished in October 2024 and the largest floating photovoltaic installation in the country. The symmetry is almost too tidy. A hole dug to feed coal-fired power stations, refilled with water, now generating electricity from sunlight on its skin. Nobody set out to make the point. It simply followed from the fact that a flat, empty, already-industrial surface was sitting there unused.
Superlatives here need care. By surface area the Cottbuser Ostsee has overtaken the Geiseltalsee in Saxony-Anhalt, itself a flooded lignite pit - but the Geiseltalsee holds around 423 million cubic metres of water against Cottbus's 126, more than three times as much in a smaller footprint. This lake is wide rather than deep, dropping to 30 metres at its lowest point and far less across most of its spread. Set beside Lake Constance, 536 square kilometres and 251 metres deep, it is a puddle. What it is instead is the newest large piece of the Lusatian Lake District, a chain of artificial lakes almost all created the same way. Ground that was excavated for a century is being filled in and let out to sailors, and Lusatia is finding out what it looks like as a place people travel to rather than take from.
The Cottbuser Ostsee lies at 51.79 N, 14.43 E, immediately north-east of Cottbus in southern Brandenburg, and it is among the easiest navigation marks in the region: a broad, raw-edged sheet of water on flat ground, the geometry of the old pit still legible around its rim, with the floating solar array reading as a distinct dark rectangle on the surface. The cooling towers of the Jänschwalde power station stand a few kilometres north and are visible much further. Best appreciated from 3,000-5,000 ft, where the lake, the Spree, the braided Spreewald channels to the north-west and the neighbouring flooded pits all read together as one post-mining landscape. Neuhausen (EDAP) is the nearest active field, about 10 km south-east of Cottbus, with a newly resurfaced 1,080 m asphalt runway; Berlin Brandenburg (EDDB) is roughly 90 km north-west. Note that Cottbus-Drewitz (EDCD), still marked on older charts about 25 km east, closed to all air traffic on 31 January 2020. Summer haze is the usual visibility limit; winter brings low stratus over the water.