From the outside, Torre Girona Chapel at the Polytechnic University of Catalonia in Barcelona looks like any one of the thousands of old churches that can be found throughout Spain, with a large cross mounted on the roof and a rose window perched above the entrance. Step through the chapel doors, however, and you won’t find any religious iconography or a congregation in prayer.

Instead, you’ll find the 25th most powerful supercomputer in the world: the MareNostrum 4.

Heralded as the “most beautiful data center in the world,” the MareNostrum supercomputer came online in 2005, but was originally hosted in a different building at the university. Meaning “our sea” in Latin, the original MareNostrum was capable of performing 42.35 teraflops—42.35 trillion operations per second—making it one of the most powerful supercomputers in Europe at the time.

Yet the MareNostrum rightly became known for its aesthetics as much as its computing power. According to Gemma Maspoch, head of communications for Barcelona Supercomputing Center, which oversees the MareNostrum facility, the decision to place the computer in a giant glass box inside a chapel was ultimately for practical reasons.

The Torre Girona chapel from the outside. Image: Barcelona Supercomputing Center

“We were in need of hundreds of square meters without columns and the capacity to support 44.5 tons of weight,” Maspoch told me in an email. “At the time there was not much available space at the university and the only room that satisfied our requirements was the Torre Girona chapel. We did not doubt it for a moment and we installed a supercomputer in it. ” Maspoch said that the chapel had been deconsecrated—the formal process by which a sacred space is converted for secular use—sometime in the 1970s. The university had used the chapel for various purposes since then, but when Barcelona Supercomputing Center was looking for a new home for the MareNostrum 3 in 2013, the chapel had fallen into disuse.

According to Maspoch, the chapel required relatively few modifications to host the supercomputer, such as reinforcing the soil around the church so that it would hold the computer’s weight and designing a glass box that would house the computer and help cool it.

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Over the past 15 years, Barcelona Supercomputing Center has continued to beef up the MareNostrum’s computing power. In 2017, the fourth iteration came online with a peak computing capacity of 11 thousand trillion operations per second (11.15 petaflops)—more than a tenfold increase in performance over the third generation MareNostrum computer.

MareNostrum 4 is spread over 48 server racks comprising a total of 3,456 nodes. A node consists of two Intel chips, each of which has 24 processors.

Although the machine cost roughly $40 million, the MareNostrum supercomputer is freely available to scientists, and has been used for research on areas as diverse as astrophysics, climate modeling, and genome research.