Hence the guesswork until you dig up the ground or drill a well.

Which on Time Team is absolutely hilarious. "I think this mottled patch indistinguishable from all the other mottled patches is a ROMAN VILLA!" And then they dig it up and it turns out to be NOT A ROMAN VILLA! AGAIN! Madame Contrivio the Mystic Psychic Lady could probably provide equally useful pointers on where to dig and she could do it on a rainy day.