Bubonic Plague victims probably..................
"Bring out your dead (Monty Python)." "And put them under the new supermarket."
That is my thinking too.
The simple fact is, in a site like Paris which has been inhabited for thousands of years, EVERYTHING is built over an old cemetery, hospital, church, dry well, or other place where people got buried many years ago.
Americans don't really "get" this because we just don't have many sites like that (Boston is one, because it's (1) old - as we count old; and (2) on a very restricted piece of ground).
“I sometimes think that never blows so red
The Rose as where some buried Caesar bled;
That every Hyacinth the Garden wears
Dropt in her Lap from some once lovely Head.”
- the Rubaiyat, translated by Edward Fitzgerald, illustrated by Edmund Sullivan.