The Tollen Dam, also known as Tollen Hydroworks, was a hydroelectric dam in Tyrus, just outside the city of Tollen. The dam was built to generate power for several cities and towns across Tyrus, including Tollen, Fort Reval, Speyer, Bronn, Ilima, and Montevado through three primary substations.
The Tollen Dam is at least a century old, having been built sometime before Imulsion's potential as a high-energy fuel source was discovered by Dr. Helen Cooper. The dam was decorated with four massive busts, representing the four then-known sources of energy: air, water, solar, and gas. The Dam was built in the wanning years of Tyrus's Monarchy and the early years of the COG Regime. Being built along a mountainous river, the Dam's primary focus was to provide the massive ammounts of inexpensive electricity needed to supply Tyrus's growing manufacturing inland region. It was eventually joined by a series of other power plants as the local economy grew. Even after the widespread application of Imulsion the Dam remained economically competitive with the "miracle" fuel.
By the time of the Locust War, the Tollen Dam was still in operation and eventually became the sole power source for the Coalition of Ordered Governments' dwindling manufacturing capacity. Upon learning that the Locust Horde planned to destroy the regional power grid, the COG called in all of its available Gears from Speyer, Tollen, and Fort Reval to defend the dam. For those Gears, this would be a suicide mission. Upon entrenching themselves within the dam, the Gears were quickly besieged by the Locust with no means of escape and no hope of reinforcement. Officially, dozens of Gears were trapped; in reality, many more had been deployed to the dam, and the COG decided to downplay their numbers to save face.
Delta Squad was dispatched to exfiltrate the dam's defenders, but Delta was unable to break through the Locust lines. The Gears gradually ran out of supplies and rations, becoming so weak they were unable to use their weapons. The Locust then overran them. Realizing the dam had fallen and a power outage was imminent, COG high command issued a work-stoppage and withdrawal order to its military manufacturing personnel in Speyer, who were then reassigned to Ilima. When the Riftworm later sank the city of Tollen, the dam survived the destruction.
Following the end of the Locust War, the dried-up reservoir behind the Tollen Dam was re-purposed as one of many burial sites for thousands of crystallized Locust corpses. Upon the emergence of the Swarm, the burial site and dam became the site of a large nest. In the morning of the day following the 25th anniversary of the Victory Day, Marcus Fenix led a reconnaissance mission to the Tollen Dam after learning that Reyna Diaz was being held in the Swarm Hive. The team managed to restore power to the dam using the last intact turbine, powered by the heavy wind as the reservoir was long dried-up though they had to reset the generators before power came back. After seeing the extent of the hive, Kait Diaz agreed to Marcus' earlier suggestion that they call for help. Rerouting the main power feed from a substation, the squad was able to restore power to Speyer, Bronn, Tollen, Ilima, Fort Reval and Montevado, allowing them to use a radio tower in Speyer to call for help.
After receiving help in the form of Damon Baird, Augustus Cole and Samantha Byrne, an assault was launched on the dam to rescue Reyna. However, she was found to be connected to the Swarm network and at her request, Kait euthanized her mother.