Avalanches of sediment along the seafloor (called turbidity currents) dominate sediment transfer into much of the deep ocean and form the largest sediment accumulations on Earth (submarine fans). These spectacular underwater flows carve some of the deepest canyons and longest channel systems found on our planet. Turbidity currents are sometimes very powerful and travel with speeds of up to 20 m/s. They can break valuable seabed telecommunications cables that now carry over 99% of global data traffic. These seabed sediment avalanches also play an important role in organic carbon transfer and burial, affecting longer-term climate change. Turbidity currents supply key nutrients and physically disturb deep-sea ecosystems, including biological diversity hot spots along submarine canyons. Despite their importance, there are remarkably few direct observations from turbidity currents in action, and they are thus poorly understood. This starkly contrasts with other major sediment transport processes, such as rivers. Research cruise JC187 on the RRS James Cook was the first of four research cruises. It revealed one of the largest channel-blocking landslides and made it possible to record the longest turbidity current ever measured, which caused two seabed telecommunications cable breaks.