Many commercially available medicines today can trace their origins to compounds found in the natural world, yet many of those natural compounds are found in rare species, often in natural environments that are now vulnerable due to human activity. Are we in danger of losing these potentially valuable resources before they are even discovered? Monty Don explores this question through a field report from the Elan Valley in mid Wales where a tree lungwort, ravished by pollution and climate change, could provide a potential cure for Creutzfeldt-Jakob disease in humans. With increased pressure from human activities in natural areas, what can be done now to ensure the survival of the unknown for future generations?