Valle de los Caídos III (Cuelgamuros), Madrid (2016). The Valley of the Fallen is the site of the largest mass grave in Spain. Built by the forced labour of political prisoners, and inaugurated on 1 April 1959, it contains the remains of 33.833 people.Valle de los Caídos II (Cuelgamuros), Madrid (2016). Rainwater has been allowed to leak through fissures in the granite cliff out of which the monument, inaugurated in 1959, was carved, flooding parts of the mass graves and leaving wooden boxes containing bones to rot and fall apart.