You're quite likely to stumble across something ancient and then you're really up whatsit in a barbed wire canoe sans paddle, because the archaeologists will be on you in a flash and put a rapid stop to your construction ambitions.
As in the case of this place: a covered arcade that's about as upmarket as Mainz gets (but doesn't really quite coupez the moutarde..) and was destined to be rather naffly named "Lothar Passage" after the alley that it covered, which in turn was named after a Holy Roman Emperor from 800-and-a-bit AD
Did they stumble upon a simple mediaeval midden or perchance a Roman shipyard from the turn of the last milennium-but-one?
No they did not.
Did they stumble upon a temple to the Goddesses Isis (his Bobness even wote a song ) and Magna Mater AND - below that - Celtic graves from 800BC ?
Yes, they bloody well did and the temple really is quite sensational, being one of only 2 north of the Alps and the only one in a presentable condition.
So they decided that an underground car park precisely in this spot wouldn't be a good idea after all and maybe we should have ruins where the cars were going to live and make a museum out of it.
Which is what they did.
Quite excellent it is, too, and it's all happening under the feet of the
(And remind me to tell you the joke sometime about the doctor and the girl with a lisp......)