Näsijärvi was once part of the Baltic. Around nine thousand years ago the land, still rebounding from the weight of the ice sheet, lifted it clear and left it landlocked. Roughly seven and a half thousand years ago the trapped water broke through to the south, cutting the channel now called Tammerkoski and falling 18.2 metres into Pyhäjärvi below. From there it runs on through the Kokemäenjoki to the Gulf of Bothnia. That drop is why Tampere exists. Eighteen metres of falling water in the middle of a forest is power, and the mills that grew along the Tammerkoski turned a set of rapids into Finland's industrial city. The lake did not shape the town's setting so much as supply its engine.

The lake is not one open sheet but three linked fjards — Näsinselkä at the Tampere end, then Koljonselkä, then Vankavesi — strung out over 44 kilometres of drowned bedrock. Bays reach off in every direction with names that map the shoreline like a gazetteer: Lielahti and Aitolahti at the southern end, Tervalahti running five kilometres northeast, Laakonselkä more than four. Between Koljonselkä and Vankavesi lies an archipelago, and just south of it the lake is crossed by the terminal moraine of Inner Finland, a ridge of debris dumped by the retreating ice. At 256 square kilometres this is Pirkanmaa's largest lake and Finland's sixteenth largest. It runs to 65.6 metres at its deepest but averages under fourteen, which is to say it is mostly shallow water with a few deep trenches let into it, holding some 3.5 cubic kilometres in all.

The steamer Kuru was built in Tampere in 1915, thirty metres long and five and a half wide, a small workhorse for a lake full of them. In 1927 a third deck was added at a Varkaus yard, five-millimetre sheet metal to carry more passengers, and nobody checked what it did to the vessel's balance. On 7 September 1929 the Kuru met a gale on Näsijärvi. The wind ran to force eight and the seas came up short and steep; three large waves struck in succession and the top-heavy steamer went over. Of roughly 160 people aboard, at least 136 drowned. They were ordinary passengers on an ordinary autumn crossing — families, workers, people going to see other people — and they died because a deck had been added to a small ship without anyone working out what it would do to her balance. It remains the deadliest inland shipping disaster in Finnish history.

One of the dead was a member of parliament. Ida Johanna Vihuri, born Hildén in 1882, had gone to work in a cotton mill at thirteen and become a trade unionist. During the civil war of 1918 she served in the Red administration; after the Battle of Tampere she was captured, sentenced to life imprisonment, and pardoned in 1920. Two years later the voters of the Häme North constituency sent her to parliament for the Social Democrats. She was travelling to a political gathering when the Kuru went down, seven years almost to the day after she took her seat. Mill child, prisoner, legislator: it is a hard life to summarise, and it ended when the Kuru rolled over. The wreck itself was raised on 22 September and returned to service by mid-October, the added cabin structures stripped out to correct the balance that had killed her passengers. The ship worked on until 1938 and was then cut down to a cargo barge. Her steam engine went to Tampere's museums in 1992 and has been on public display at Mustalahti harbour, on the Näsijärvi shore, since 2013.

For most of the twentieth century the lake paid for the industry it powered. Pulp and paper works discharged into it until the mills began closing between 1983 and 2008, and the water has recovered remarkably: current assessments rate it excellent on physical and chemical measures and good biologically. Pike, perch, whitefish and pikeperch support the fishery; trout and salmon draw the anglers. Above it all stands Näsinneula, opened in 1971 to Pekka Ilveskoski's design — 168 metres, the tallest free-standing structure in Finland, with a restaurant at 124 metres that takes forty-five minutes to complete one revolution. Werner Holmberg painted a storm on this lake in 1860. From the top of the needle, on the right afternoon, you can watch one arrive.