Tuonela, along with the other planets in its system, was first surveyed by the Systems Alliance in late 2156. Soon after the initial surveys and the discovery of its vast mineral resources, more teams were dispatched to evaluate Tuonela as a potential mining prospect. At first, due to the inhospitable surface conditions, the planet was seen only as a robo-mining target with a very minor human presence on the surface. In their rush to claim the world and in search of much wanted osmium deposits, the Alliance had already posted an automated drilling mission to the south polar regions before the planetary survey was complete. These operations lasted barely more than a year as Tuonela's large subterranean tunnels and caverns were discovered. Plans for subterranean geothermally-powered habitats were swiftly drawn up and the automated surface drills were dismantled soon after. MAKO vehicles and tractors flooded the vast underground network of tubes, continuously cataloging the numerous of exposed mineral veins in them.
When the news of the findings on Tuonela spread on Earth, a new 'gold rush' broke out amongst the members of the lower classes. Many different companies promised safe transport and housing to the would be miners and fortune makers as well as their families. The main settlement in the 'St. Peters Cavern' grew rapidly and soon gained the title of a city as well as the reputation of a 'true marvel of colonial engineering' as its builders had successfully overcome a number of construction issues during early development.
When contracting new colonists, companies usually marketed Tuonela to the countless desperate lower-class citizens of Earth that were seeking empoyment in job fairs, or generally a new life in the colonies, but did not fullfill the various criteria required for citizenship approval by the authorities of the wealthier Alliance worlds. Various low-to-moderate-skill positions were offered, such as miners, plummers, maintenance workers, and many others. As a result, Tuonela's demographic is comprised mostly of Earth citizens with poor backgrounds, hailing from the more underdeveloped parts of the human homeworld. Though local colonial authorities do their best to maintain a decent standard of living for the colonists, Tuonela still has one of the lower Human Development Indeces amongst Alliance worlds.
During the war, the entire Carina Nebula was completely cut off from the rest of the galaxy by its relay in the Byzantium System. Travel in and out of the cluster was completely halted and all communications went silent. Though the Reapers ignored the Nebula due to its relative isolation and insignificance, barely a month into the war, Cerberus made a move within the cluster. Their forces first occupied the hub world of Constantine and then proceeded to take control of the rest of the colonies in the area, including Tuonela. Cerberus took advantage of the colonists' fear and uncertainty that followed the communications blackout, presenting themselves as saviours and protectors of the colony, while at the same time spreading stories about the complete destruction of the Systems Alliance. Most of the colonists, the majority of whom weren't particularly familiar with the exact nature of Cerberus and its history with the Alliance, eventually trusted and sided with Cerberus, voluntarily signing themselves up in many cases. This ultimately lead to either the indocrination and assimilation or enslavement of most of the some thirty thousand human colonists. The handful of survivors that were left retreated deeper into the cold tunnels and caverns of Tuonela, hidding from the Cerberus troops but often engaging them in guerilla warfare and maintaining control of key tunnel junctions and mineral veins. In the meantime, Cerberus was taking control of most of the remaining mining equipment and refining facilities in the Tuonelan tunnels and utilised the collected resources for the benefit of their forces. Those colonists that were willingly recruited by Cerberus or had proven suitable for combat were assimilated and converted into indoctrinated troops while the rest, who refused to submit and did not have time to escape into the caverns, were forced to harsh labour deep in the mines.
With the conclusion of the Reaper War and the disappearence of Cerberus forces from the Carina Nebula at large, the few remaining survivors of Tuonela re-emerged from the tunnels and reclaimed St. Peters Cavern. This marked the beginning of a several-year-long struggle to keep the badly damaged climate control and power systems operational and survive until the world's eventual reconnection to the rest of the galaxy.