The Crossrail project is a new underground railway running east–west through the heart of London. It will connect with 110 km of new or upgraded sections of surface rail to Reading, Shenfield and Abbey Wood. Crossrail co-ordinated ground investigations for the central tunnelled sections began during the 1990s and continued to 2011 according to a strategy that was based on a review of other major tunnelling projects and best practice. The level of detail in the investigations varied according to local geological complexity and design requirements. However, the state of knowledge on the geological structure is hampered by London's long history, leading to assumptions on the level of complexity or simplicity. Drift-filled hollows and Lambeth Group sand channels were known features presenting risks to excavations but relationships with faulting were unknown or unproved. The emerging geological knowledge was developed through multiphase investigations and collation of third-party data to allow far-field considerations. The risk profile for tunnel boring machine tunnels is lower than that for sprayed concrete lined tunnels but the risk for both increases with increasing proximity to pre-existing infrastructure. Ground movement assessments become critical and mitigation of movements can be significantly affected by unexpected variations in ground conditions.