Green spaces have always been important in cities. As the new urban classes of early Bronze-age Egypt developed new cultures, gardens were planted amongst the dense clusters of buildings that provided homes away from the fields and spaces for commercial and cultural exchange. Five thousand years later, we now know scientifically, that cities lacking in green are damaging to health. To calibrate the greening of cities, what new discoveries can be made about the psycho-physiological and behavioural interactions of humans with nature? Can ‘dose response curves’ be measured? Is it the absolute quantity or frequency or texture of green that matters most? Should green space all be allocated in one big central park or distributed in multiple ‘pocket parks’ or somewhere in between? What size distribution of parks within a city yields optimal happiness and mental health and induces more walking, active commuting and external recreation? If we can discover exactly how the human sensing of nature converts into mental and physical health, can we replicate the effects artificially in virtual green experiences? Might future cities have pop-in nature booths? Might urban spaces be designed physically and virtually to give the same experience as being in the Alps?