The small Estonian town of Sillamäe is, today, easy to track down and even features in tourism guides. But during the Soviet era, it was a secret town, rarely depicted on maps and only referred to using codes like Moscow 500 and Leningrad 1.

In the 19th century it was a small resort town that celebrities of the time visited, mostly from Russia, including Tchaikovsky and Pavlov. But right after World War II the town changed forever. Due to the building of a uranium enrichment plant in the region, it became off-limits to anyone other than those directly linked to its operations.

Constructed in the 1940s through the efforts of several thousand prisoners, the secret town was home to around 3,000 residents. Since it was a “closed town” with little contact to the outside world, the government took special measures to make it a habitable and interesting place for engineers working at the factory.

Even the architecture of Sillamäe is different than typical Soviet style; the buildings were built in the style of Stalinist neoclassicism. The main example is the town hall building that looks just like a church but has never served as one.

In the beginning, the uranium was mined locally, but was later imported from other parts of Europe. In the five decades of its functioning, the plant in Sillamäe refined over 100,000 tons of uranium, which was in turn used in 70,000 nuclear weapons, including the Soviet Union’s very first nuclear bomb.

This extended period of radioactivity did not leave the landscape unharmed and even before the fall of the Soviet Union, the operations were winding down at the plant. After the collapse, it was liberated along with the rest of Estonia, and the factory was adapted to process rare metals and particles.

Sillamäe remains a enclave where ethnic Russians are a majority, the street names are often in Russian, and the Communist symbols like sickles, hammers, and stars can be see woven into the decor of buildings—a reminder of the Soviet Union’s once expansive power and influence in the region.