Lagos faces increasingly severe annual flooding, exacerbated by sea level rise and subsidence. In 2021, floods again submerged vehicles and houses, displacing thousands from their homes. The demand for sand in construction materials drives illegal sand mining and erosion on the coast of Nigeria, degrading ecosystems and increasing flood risk. This technical background report for the 2021/2022 edition of the Interconnected Disaster Risks report analyses the root causes, drivers, impacts and potential solutions for the Lagos floods through a forensic analysis of academic literature, media articles and expert interviews.