Newgrange

Built around 3200 BCE, Newgrange is a large circular mound 85 metres in diameter containing a 19-metre passage leading to a cruciform chamber. Its most extraordinary feature is the roof-box above the entrance: a precisely aligned aperture that admits a shaft of sunlight into the chamber at sunrise only during the five days surrounding the winter solstice, illuminating the central basin stone for 17 minutes.

This alignment could not have been accidental given the precision required — the tolerances are within a fraction of a degree. The engineering required moving over 200,000 tonnes of material with Neolithic tools. In Irish mythology, Newgrange (Sí an Bhrú, 'the Mansion of the Boyne') was the home of the Dagda and the site of the conception of the hero Aengus. The surrounding complex at Brú na Bóinne is a UNESCO World Heritage Site.