A particle accelerator is a scientific tool that accelerates subatomic particles to extremely high speeds, often near the speed of light, to collide them and study their basic properties. During the Cold War, the Soviet Union undertook one of its most ambitious engineering projects with the Protvino tunnel, an underground ring with a 21-kilometer circumference, located about 100 kilometers south of Moscow. Completed in 1994, the tunnel was meant to house the UNK, a proton-proton collider with an energy capacity of 3 TeV, which would have required 2,500 superconducting magnets. However, the tunnel has remained empty for over 30 years, with no magnets ever installed. The Protvino tunnel was built between 1983 and 1994 using open excavation through 26 access shafts, some as deep as 60 meters. It has a diameter of 5 meters and is longer than the circular line of the Moscow Metro. The location was chosen for its geological stability, as Protvino lies on a former dried seabed, making it suitable for large underground structures. The final 2.7 kilometers of the tunnel were the most challenging to complete due to water infiltration, which delayed the project until 1994. The UNK project aimed to create a proton-proton collider with a 3 TeV energy capacity, a level that could have rivaled or exceeded the capabilities of CERN’s Large Hadron Collider in Europe. This would have required 2,500 superconducting magnets, each weighing about 10 tons. However, the dissolution of the USSR in the early 1990s led to a lack of funding, leaving only a small portion of the accelerator infrastructure built and just a few dozen magnets produced—far short of the 2,500 needed for the machine’s full operation. After its completion, the tunnel was sealed off in 1994 and officially put on hold in 1998 by the Russian agency MINATOM. Limited work, such as lighting and ventilation in the underground areas, continued until 2013, after which the project was completely abandoned. Today, the tunnel lies silent, buried under layers of earth and covered by forests. Some of the scientists who worked on the UNK project have since moved on to other major accelerator projects around the world. Recently, there has been renewed interest in repurposing the tunnel for a future muon collider, a type of particle accelerator that could offer new insights into physics. While the future of this idea remains uncertain, it could give new purpose to the 21 kilometers of tunnel built in near silence.