Why Do We Feel Giddiness from a Height

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When we walk on land, we are quite comfortable with our surrounddings and never get scared. If we go higher on a mountain, even then we need the same width to walk and don't feel anything strange. But if we have to walk on a strip more than wide enough to walk at some height, say 100 feet, our blood pressure rises and we have a creepy feeling beneath our sole. Why does it happen? Why do we feel giddiness when we look down from a multi-storey building, whereas it doesn't happen when we look down from the window of an aeroplane, though the height is normally more than 30,000 ft.