Madrid’s Metro system is a world unto itself: 294 kilometers of track, 301 stations and 13 lines, through which travel some 2,318 trains each day, carrying around two million passengers. There are also four tunnels that are not in use: one, at Chamartín, the high-speed train station in the north of the city, was built as the starting point for a north-south axis that was never completed.

Two more, one close to Moncloa in the west of the city and another between Nuevos Ministerios and Avenida de Ámerica to the north, may be familiar to older residents. The fourth is an annex of Goya, in the upscale Salamanca neighborhood, which was built in the 1930s, but has been abandoned for the last six decades.

Hidden in plain sight on the northbound platform of Line 2 at Goya station is a metal door, and behind it a passageway that, until 1958, led to a one-way track to nearby Diego de León station. It ceased to be used after the inauguration of Line 4, which originally connected Diego de León with Argüelles in the west of the city.

It’s still possible to catch a glimpse of the tunnel aboard trains arriving at Goya from Lista: it’s on the left, just before the entrance to the station. Until the 1990s, its platform was used as a storage area. In its day, this was where a workforce of up to 60 men would carry bags filled with coins from the ticket machines.

Line 3 originally ran between Sol and Embajadores, and was opened on August 9, 1936, less than a month after the start of the Spanish civil war

The 170-meter-long tunnel is well maintained and is a perfect example of the trench method used to build many of the early stretches of Madrid’s Metro system. Along one side is a beautifully preserved pumping well, along with other air vents and the cavities dug into the side of the tunnel where maintenance staff would shelter as trains passed by.

Line 3 originally ran between Sol and Embajadores, and was opened on August 9, 1936, less than a month after the start of the Spanish Civil War. It was extended westward to Argüelles in 1941 and south to Legazpi a decade later. The final stretch between Argüelles and Moncloa took another 12 years to complete.

In 2006, lines 3 and 6 were incorporated into the Moncloa transport hub. The new Line 3 station is a little further south than its predecessor, which has been hidden behind a wall. But 550 meters of the old track are hidden in a stretch that can only be accessed by Metro staff via a door on the platform.

When Line 1 was extended to Chamartín in 2007, the plan was to create a new stretch of Line 11. This would have involved building some 15 kilometers of new tunnel. The plans were dropped the following year after Spain’s economy went into recession. All that is left is about half-a-kilometer of tunnel running northward to nowhere.

Until Madrid City Hall decides whether to resume work, the platform at Chamartín has been asphalted and is now used by vehicle manufacturers to launch new models.

At 3am, Avenida de América station is empty, making it possible to clamber down on to the tracks, where, 250 meters to the west, the extension begun in 1999 toward Pitis opens up. To the right is another tunnel.

The track was in use between 1986 and 1996, but today is used for maintenance.

Despite the abandonment of the line, there are echoes of human activity everywhere. On one of the walls, someone has written in large latters the word “Negredo.” Who knows whether it is a homage to a family surname, or testament of admiration of the Valencia soccer player. It’s just another mystery to add to the stories of the forgotten tunnels of the Madrid Metro.