The Valley of the Fallen is a Civil War memorial and underground basilica built under Franco between 1940 and 1959, located about 10 kilometres north of El Escorial in the Guadarrama mountains. A 150-metre granite cross tops the hill above; the basilica itself is carved into the rock below, 260 metres long and 40 metres high. It was built partly by Republican prisoners of war using forced labour — a fact that makes the site deeply controversial.
The site is currently undergoing reinterpretation. Following Franco's exhumation in 2019, the Spanish government has been working on a plan to convert the monument from a Francoist shrine to a memorial to all Civil War victims. Access arrangements and opening hours have changed several times in recent years; check current status before visiting. The setting — a wild mountain valley of pine and granite — is striking regardless of your views on the politics.