Completed all the way back in 1935, this is the Grande Dixence Dam which holds back the Lac des Dix, a 4 km long lake in the Val d’Hérens, Switzerland. The water mainly comes from the summer melt of glaciers, and is piped here via a system of water supply tunnels over 100 km long.

What’s really interesting is that in summer the Lac des Dix can be up to 284 metres deep¹ - thanks to the fact that at 285 metres tall, the Grande Dixence is the world’s tallest dam.

Update: It turns out that this is actually the world’s 2nd tallest dam. See my comment for more info. Thanks to Gary for the correction.

Thanks to Joel.