In 'Le Berbier de Seville' [Act 2 Scene 8] the phrase 'growing before your very eyes' dates this type of expression at least back to 1772, and in that same scene the words piano and pianissimo are used in their literal latin root sense to mean quietly.
Verum, veritas, and verily are all words closely related to very, and in the phrases
'he ate the very last cookie', and 'a place to call your very own',
very means 'in truth' or 'really'. 'can I really keep it?' (the puppy) means
'confirm this is not a trick' or 'is it true i can keep it'.
so, before your very eyes, and beneath your very feet, seem to apply
the sentiment of 'can it be true' by imitating the structure of those two phrases (very last, very own).
the proper way to phrase it would be 'very happening beneath your feet' (in truth, in reality)
but either by mistake or aesthetics 'happening beneath your very feet'
won out over time and usage.