There is a military secret across the river

est.reading time: 4 min

Hidden in plain sight

During our running tours, while crossing the Douro on one of its bridges, we’re usually too busy admiring the sunrise to the east. But southeast of our position, just a couple of hundred meters away, lies the Geophysical Observatory (Instituto Geofísico do Porto)—a site that once played a silent, high-stakes role in global geopolitics.

A weather station since 1885

Perched at the top of the hill, this structure has been providing weather data since the nineteenth century. More recently, it has also been feeding our weather apps, which have become indispensable for checking if the Porto morrinha (morning drizzle) will ruin our training session.

Yet, this observatory—historically known as Station PTO—was once a vital node in a secretive project: the 'World-Wide Standardized Seismograph Network' (WWSSN). Launched in the early 1960s, the network was marketed as a scientific endeavor to study plate tectonics...

Main building of Instituto Geofísico do Porto(Geophysical Observatory),
​where weather data is collected. Source: OpenHousePorto

The bunker holds the secret

In reality, the WWSSN was a U.S.-led effort to detect and monitor underground nuclear tests conducted by the Soviet Union. Because the observatory (and most of Porto's urban area) sits on stable granite, it provided the perfect environment for high-precision instruments to detect tremors from thousands of miles away.

A few meters from the main building, this bunker still houses the seismic monitors, funded by the Eisenhower administration.

What exactly was WWSSN?

The WWSSN arose from a political concern. In the 1950s, concerns about radioactive fallout from above-ground testing of nuclear weapons prompted the leadership of the three leading nuclear nations (USA, Soviet Union, and UK) to ban further testing of nuclear weapons.
However, there was a hitch. The United States would not agree to banning nuclear tests if there was no capability to detect and identify violations: for smaller underground tests, seismology was not sufficiently developed to have that capability...

The Eisenhower Administration therefore decided to improve the USA's seismic detection abilities by designing and building what became the WWSSN. Performance specifications and a request for proposals were published in November 1960, a contract awarded in early 1961, and the first station was installed in New Mexico in 1961. An additional 89 stations were installed by the end of 1963, one of them right here, in that bunker in Porto (or Vila Nova de Gaia, to be more precise).

Even though WWSSN's  main purpose was to monitor any possible nuclear tests, the unprecedented collection of high quality seismic data helped scientists better understand the mechanisms of earthquakes and the structure of the Earth's crust, and contributed to the development of plate tectonic theory.

Fast forward to 2026.

Today, the observatory deals with a more urban kind of "noise." Since 2005, the Metro’s Yellow Line has crossed the top deck of the iron bridge, which is anchored to the same solid granite hill. Every time a multi-ton train rattles across the top deck of the iron bridge, the short-period sensors record a "fuzzy" spike. To a 1960s spy, it might have looked like a tiny earthquake; to a modern seismologist, it’s just the 3:15 PM commute to Trindade. 

It turns out that the seismic "footprint" of public transport is very predictable—unlike your running pace after a night of Francesinhas and Port wine :)

The metro system in Porto uses the upper deck of Luis I bridge since 2005

If you want to visit this site, which is owned by the University of Porto, you can.

Porto Secret Spots offers guided tours of the space on Saturdays, not only explaining the military 'secret' but also revealing other features of a building that—and we can vouch for this—has one of the best views in the city!

source: ​https://igup.fc.up.pt/en/historia/

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