A tonne of ore from a gold mine yields around 5 grams of gold. A tonne of discarded mobile phones yields between 150 and 300 grams, a comparison that gave the practice its name (Nanjyo, 1988). Japan imports almost all of its metals and built its recovery system accordingly; the medals for the Tokyo 2020 Olympics were cast entirely from 78,985 tonnes of donated devices. The idea spread to other secondary resources: phosphorus from sewage sludge, which Germany has required to be recovered since 2017; rare-earth magnets from motors; copper from decommissioned cable. Industrial ecologists began to measure the anthropogenic stock, the total mass held in human-made things, and found that most of it was buildings. The EU Horizon 2020 project BAMB, Buildings as Material Banks (2015 to 2019), set out to make the building stock legible as a deposit: a passport for every component, protocols for reversible construction, and a way of reading a city as inventory. The framing is new, but the practice is old. For centuries after the fall of Rome, the city's ruins served as a quarry, and the columns reused in new churches are what architectural history calls spolia.

Mining a city means knowing what is stored and where, knowing what is about to become available, and getting it out in one piece.

K.118 (Baubüro in situ, 2021) added three storeys to an industrial hall in Winterthur. The steel frame came from a distribution centre in Basel, the external stair from an office in Zurich, the windows and facade panels from buildings across Switzerland, and the design changed as the team found what it could find. The project reported embodied-carbon savings of about 60 percent against a conventional new build. Resource Rows (Lendager Group, 2019) in Copenhagen solved the brick problem differently. Because cement mortar makes bricks impossible to separate, whole panels of brickwork were sawn out of demolished buildings, including a former Carlsberg brewery, and set into the facades of 92 townhouses. At Grand Parc in Bordeaux (Lacaton & Vassal, 2017), winter gardens were added to 530 social-housing flats instead of demolishing them, at about a third of the cost of replacement, under the rule: never demolish, always add, transform, and reuse. These projects are still exceptions. They show what recovered material can do when the available inventory is established before the design is fixed.

The first dependency is information. A material passport records what a building contains, how it is connected, and how it has performed, turning an unknown deposit into a documented one. Madaster links passports to BIM models, a digital twin keeps them current, and BAMB produced prototype passport standards. Without the record, every recovered beam is an unknown and has to be tested from scratch. Several practical barriers remain. Codes and warranties assume new material, so a reclaimed steel section must be sampled and re-certified before an engineer will sign for it. Insurers have no loss data on reused components. Recovered material needs storage and cleaning before it can be sold, and deconstruction costs more than demolition while landfill fees stay low and virgin material stays cheap. Policy is moving some of this: pre-demolition audits are mandatory in several EU states, and Denmark and the Netherlands cap embodied carbon in new buildings. A circular economy in construction will need regional reprocessing networks, a kind of distributed manufacturing, to close the distance between where material is released and where it is needed. The material is already in cities. What is still missing is the infrastructure to recover it at scale.