You need to be very persistant with them, though watch out where you place them - the scafolding helps as a guide so that you don't get too close and produce another large batch of children if you are high on population - alwasy reserve at 89 so that you have room for that one special villager when he/she is born. I find it interesting though, that nothing happens after they uncover it - they don't take it, no tech points go zooming upward - I originally thought that once you got that treasure, you would have a lot of points on your researching table.