Matt Baker & Sammi Kinghorn are in Bannau Brycheiniog National Park, where the power of the water that runs through its valleys shapes the landscape both above & below ground.  Matt joins a local caving team who are protecting the UK's deepest cave system, while Sammi visits a 200-year-old mill still driven by the force of the water. From one of the wettest corners of the UK to one of the driest, Adam Henson is in Lincolnshire, where a farmer is taking a gamble on a classic Mediterranean crop that thrives in dry conditions; olives.