Inside the monastery's Chapter House — a fourteenth-century octagonal room with one of the boldest unsupported stone vaults in medieval architecture — lies the Tomb of the Unknown Soldier, the Portuguese equivalent of the eternal flame at the Arc de Triomphe. The vault was so audacious for its time that it twice collapsed during construction; the third attempt, built only with prisoners condemned to death, held.
Two soldiers stand silent watch in dress uniform on either side of the tomb at all times, day and night. The guard change happens hourly and is brisk rather than ceremonial. Visitors enter quietly, walk a circuit, and leave — there is no commentary, no signage beyond a small bronze plaque. The atmosphere in the cold stone vault is genuinely affecting.