During excavation of the twin tunnels on Slovenia's new Divača–Koper railway line, 91 previously unknown karst caves were discovered – some hundreds of metres long, others permanently flooded or filled with unstable clay and collapsed blocks. This webinar looks at how the design team responded in real time: what the ground actually revealed, how the caves were investigated and modelled in 3D, and how four large reinforced concrete arch bridges with spans of 25 to 45 metres were designed and built to carry the tunnels across the voids.