In Finland, kindergartens are exposing children to more mud, wild plants and moss - and finding changes to their health that show how crucial biodiversity is to wellbeing. At Humpula daycare centre in Lahti, north of Helsinki, children are encouraged to get muddy. Across Finland, 43 daycare centres have been awarded a total of €1m (£830,000) to rewild yards and to increase children's exposure to the microscopic biodiversity " such as bacteria and fungi " that lives in nature. We already know that access to the outdoors is important for children and their development. But this study goes one step further. It is part of a growing body of research linking two layers of biodiversity. There is the outer layer " the more familiar vision of biodiversity, made up of soil, water, plants, animals and microbial life, that lives in the forest, playground (or any other environment). And then there is the inner layer: the biodiversity that lives within and upon the human body, including the gut, skin and airways. Increasingly, scientists are learning that our health is intimately linked to our surroundings, and to the ecological health of the world around us. The plants, dead wood and soil in the daycare centre have all been specially selected for their rich micro-biodiversity. They have also dug up and imported a giant live carpet of forest floor, 20-40cm deep and 10 metres square. It has blueberries, lingonberries and moss growing on it, to encourage the children to forage, find bugs and learn about nature.