The Seegrotte at Hinterbrühl holds 20 million litres of water inside a former gypsum mine, making it the largest accessible subterranean lake in Europe. The cavern flooded in 1912 after an explosive charge ruptured an underground aquifer, filling the galleries in three days and ending 250 years of mining. By 1932 the site had reopened as a tourist attraction, and during the Second World War it became a covert assembly hall for Heinkel He 162 fighter jets, built by forced labour beneath the forest canopy. Today the lake stretches 6,200 square metres under vaulted ceilings that reach twelve metres, lit by submerged floodlights that turn the water an unearthly jade. Vienna Woods wine tours pair the underground lake with visits to family-run cellars in the surrounding Thermenregion and southern Weinviertel appellations. The limestone bedrock that shaped the Seegrotte also defines the terroir of the vineyards, where Grüner Veltliner, Weissburgunder, and Zierfandler vines climb south-facing slopes above the Mödling valley. Many estates maintain press houses and tasting rooms in converted farmsteads, some dating to the eighteenth century, and serve wines alongside cold cuts, spreads, and freshly baked bread in the Heuriger tradition. Combined itineraries typically allocate two hours for the Seegrotte visit — including the guided walk through the mining galleries and the boat crossing — and between ninety minutes and three hours for vineyard stops, depending on the number of estates included. Most half-day excursions depart Vienna mid-morning and return by early evening, covering the twenty-kilometre southbound route through the Wienerwald along the B11 or via Perchtoldsdorf. The forest itself is a UNESCO Biosphere Reserve, dense with beech and oak, and the roads that thread it offer glimpses of the Cistercian abbey at Heiligenkreuz and the ruins of Burg Mödling. Tours that combine the subterranean lake with local wine experiences provide a concentrated cross-section of the region's geology, wartime history, and viticultural heritage within a single afternoon.