Anyone that looks out the plane window during arrival or take off at the Adolfo Suárez Madrid–Barajas Airport will see the huge white cross painted on a hill right beside the runway. It gives some passengers a feeling of unease, as some believe it to be a plane crash memorial. In fact the cross is for victims of a different sort of tragedy.

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The Spanish Civil War, fought between General Francisco Franco’s Nationalists and left-wing Republicans, lasted from 1936 to 1939. Both sides committed grave atrocities during the war, one of the worst of which became known as the“Paracuellos Massacres.”

In the Battle of Madrid in 1936, the Nationalists attacked civilians with airstrikes from German bombers. At the same time the Republicans (supported by the Russians) incarcerated thousands of political prisoners in Madrid, among them civilians, Catholic priests and soldiers. As the Nationalist troops approached, the Republicans felt pressure to dispose of their enemies. Their solution was mass executions. Starting in the early morning hours of November 7, 1936 prisoners were informed that they would be released from imprisonment in groups. They were brought to the fields near “Paracuellos del Jarama,” where they were shot and buried in mass graves. The executions continued until December 4.

How many Spaniards were killed at the hands of their fellow countrymen is unknown. Numbers vary largely among historians. While some say it was a thousand in total, others say it was about a thousand in the first two days. Most numbers vary between 2,000 and 5,000. The magazine El Alcazar in 1977 put the number at 12,000. In the same year, César Vidal’s Matanzas en el Madrid Republicano included a list of 12,000 victim names. An official number will never be known, as most of the bodies cannot be found.

The white cross visible from the runway of Madrid Airport marks the site of the Paracuellos Massacres. What can’t be seen from the runway is the Cementerio de los Mártires at the foot of the hill, the Paracuellos Civil War Cemetery. Six hundred crosses mark the graves of the bodies that were discovered, and stand in memory of the many more whose fate is unknown.