Clementinum

The Clementinum is the second-largest complex of buildings in Prague after the castle, and most people walk past it without realizing what's inside. This sprawling compound — once a Jesuit college, now the Czech National Library — covers two hectares in the heart of the Old Town and contains a Baroque library hall so beautiful that photographs of it routinely go viral with captions like "the most beautiful library in the world."

The Jesuits arrived in Prague in 1556 and spent the next two centuries transforming a medieval Dominican monastery into an educational and intellectual powerhouse. By the time the Jesuit order was dissolved in 1773, the Clementinum had swallowed up 32 houses, three churches, and two gardens. In 1622, they absorbed the library of Charles University — one of the oldest universities in the world, founded in 1348 — and never gave it back.

The Baroque Library Hall, built in 1722 by Kilian Ignac Dientzenhofer, is the showstopper. The ceiling frescoes by Johann Hiebel glorify science and art, the shelves hold 20,000 theological volumes, and the collection of antique globes is mesmerizing. You can only view the hall from the doorway — no walking inside — which somehow makes it more tantalizing.

But the Clementinum's greatest hidden treasure might be its astronomical tower. Since 1775, scientists here have been recording daily meteorological measurements without interruption — the longest continuous weather data series in Europe. For 250 years, someone has climbed that tower every single day and written down the temperature, barometric pressure, and sky conditions. That unbroken chain of data is invaluable to climate science.