Purely out of curiosity; Would a Cohen have to be careful when walking around Paris because of the Catacombs (an underground network of tombs where an estimated 6 million people's bones are buried), which are located directly underneath parts of the city?
The question is not for a problem of touching a dead but for a problem of Maahil (to be over the dead). Tum'a Bokaat Veola (uncleanness erupt upward).
- In some conditions, to walk over a buried dead makes the walker unclean,
- In some conditions, he remain clean.
- Dead has laws of Ohel, but not all deads, following some Tanayim.
See Gemara Brachot 19b:
תא שמע, דאמר רבי אלעזר בר צדוק: מדלגין היינו על גבי ארונות של מתים לקראת מלכי ישראל, ולא לקראת מלכי ישראל בלבד אמרו אלא אפילו לקראת מלכי אומות העולם, שאם יזכה - יבחין בין מלכי ישראל למלכי אומות העולם. אמאי? לימא: אין חכמה ואין תבונה ואין עצה לנגד ה' כדרבא, דאמר רבא: דבר תורה, אהל, כל שיש בו חלל טפח - חוצץ בפני הטומאה, ושאין בו חלל טפח - אינו חוצץ בפני הטומאה, ורוב ארונות יש בהן חלל טפח, וגזרו על שיש בהן משום שאין בהן, ומשום כבוד מלכים לא גזרו בהו רבנן.
Come and hear; for R' Eleazar B' Zadok said: We used to leap over coffins containing bodies to see the Israelite kings. Nor did they mean this to apply only to Israelite kings, but also to heathen kings, so that if he should be privileged [to live to the time of the Messiah], he should be able to distinguish between the Israelite and the heathen kings. Why so? Let us say, 'There is no wisdom and no understanding and no counsel before the Lord'? - [It is in accord with the dictum of Raba; for Raba said: It is a rule of the Torah that a 'tent' which has a hollow space of a handbreadth forms a partition against uncleanness, but if it has not a hollow space of a handbreadth it forms no partition against uncleanness. Now most coffins have a space of a handbreadth, and [the Rabbis] decreed that those which had such a space [should form no partition] for fear they should be confused with those which had no space, but where respect to kings was involved they did not enforce the decree.
On reading this text, we can notice 3 rules:
- Buried coffins (a dead is inside) make unclean the person who passes over;
- If the distance between the seats of the coffin and the body is at least one handbreadth, the uncleanness cannot erupt upward.
- A rabbinic decree prohibit to pass over a coffin, despite that this condition is fulfilled, to avoid confusion between coffin with a sufficient hollow space and coffins without a sufficient hollow space.
We can learn important details in Tosfot (paragraph commencing "Rov Aronot"):
- a container of a dead person, which is fully trained cannot be a partition against uncleanness.
- If the container fulfill the condition 2 of the above list, and is not fully closed, it is a partition against uncleanness. Tosfot in BB 100b explain that tombs was with hollow space of a handbreadth and was opened at the end. (Tosfot in Baba Batra said this in name of Rivam).
Catacombs are tombs with an hight roof. But it seems that Miderababan a prohibition remains.
But regarding the fact that most deaths are not Jews and that Rambam rules as the Tana which said that non-Jew has not "tent uncleanness". But Shulchan Aruch 372, 2 and Rema advise to stringency.
So it seems a rabbinic prohibition for Cohanim to walk over Catatombs. For some mitsvot the rabbinical prohibition is sometimes canceled see Gemara and tosfot in Brachot 19b. See also SA YD 362 for the psak.
Note: this is only an introduction to the subject, not a real conclusion.