An entire street in a London neighborhood sank slightly overnight, prompting local authorities to investigate the cause of the subsidence. During the investigation, municipal workers discovered a network of tunnels that were not marked on any official maps. These tunnels were the result of a resident who had been digging under his home for decades, using only a shovel and a bucket, without any permits or professional help. The man had inherited the house in the 1960s and began excavating the soil in his garden year after year. Over time, the underground maze extended beyond his property, slipping under the street and the foundations of neighboring houses. The man had been digging from the 1960s through the 2000s, without the use of engineers or support structures. The underground network caused sidewalks to collapse in 2001. To assess the damage, municipal services used ultrasonic scanners to map the cavities before filling them with concrete. The subsidence revealed decades of secret work. For a long time, neighbors joked about this unusual behavior, even expecting to see the man emerge from the floor of their kitchens. However, the joke turned serious when sidewalks above the tunnels began to collapse in 2001, creating a large hole that extended into the street. The risks were not just visual. The man had accidentally cut off electricity for the entire street by touching a 450-volt underground cable during his digging. For years, neighbors had raised concerns with the local council, but they received no clear answers. One neighbor had alerted the city six years before the incident, only to be told six weeks prior that the house was structurally sound. The underground story told a different tale. Digging without support structures highlights a basic principle of construction: buildings are designed to rest on ground that is calculated to support their weight. Removing earth underneath a structure is like removing the support beneath it, causing the weight to shift to other parts of the ground. This can lead to uneven settling, known in geotechnics as differential settlement. This phenomenon can cause cracks in walls, doors that no longer close, and floors that tilt. The same principle applies to roads and pipelines, which can eventually collapse if the ground beneath them is too weak. Predicting the exact moment of failure is nearly impossible. Faced with an unpredictable underground, technical services had no choice but to map the unknown. After years of complaints, the local council finally used ultrasonic scanners to assess the damage. This technique allows for detecting cavities without excavating the entire area, by sending waves into the ground to identify voids. The diagnosis confirmed the engineers' fears: active ground movement that could worsen at any time. The council then temporarily evicted the occupant and filled the tunnels with concrete. An inspector summed up the situation with a memorable remark: the man had “lucky that a London bus was not in his garden.” This exaggerated image highlighted the level of risk the neighbors faced. In cases where a private individual weakens public roads, the distribution of costs generally follows a consistent logic in British law, as in most European countries. The part of the underground network under private property remains the responsibility of the owner, who must fund the safety work on their own land. However, the community intervenes and funds repairs once the danger affects roads, public utilities, or the safety of others, even if they later seek to recover some of the costs from the responsible party. In practice, this recovery is often only partial. A homeowner who has already weakened their property through years of excavation rarely has the means to repay the full cost of a major safety operation, and home insurance typically excludes damage from undeclared work. Despite the risks, the house did not disappear. Later bought and restored with the help of an architect, it even received the Best House Award at the New London Awards in 2021. The former tunnels were transformed into a habitable basement. This outcome serves as a reminder that a clandestine construction site can, years later, become an architectural argument rather than a simple disaster.