> Paris is very complicated underground. Besides the quarries there are the sewers, catacombs, some underground lakes, reservoirs, etc. It is definitely a complicating factor when planning new metro tunnels.

I wonder if the French are considering digging underneath all those obstacles? Seems to be the way London has gone with the Elizabeth Line. Although that may still be shallower than Paris' quarries and lakes.

> I wonder if the French are considering digging underneath all those obstacles

Paris has been doing it for decades. All new lines in Paris (such as metro line 14, opened in 1998, RER E) have been dug by tunnel boring machines, and are at a depth below 20m. Same goes for all new lines outside of Paris proper, like most of the Grand Paris Express 200km+ new lines.

> I wonder if the French are considering digging underneath all those obstacles?

The problem is the Seine phreatic zone, which starts usually between 15 to 25m below the surface. Some GRS galleries are actually completely inundated and others have a level of water that varies between the seasons.

In order to have some metro going underneath the Seine river, they had to freeze it first. It is not an easy task, so there must be a real advantage to going under the Seine.

Tim-like pedantry corner: the quarries and the catacumbs are actually the same thing. Catacumbs are parts of the quarries that have been turned to ossuaries, and are reachable from the wider quarry network.

> You’d think exploring the closed parts of the catacombs would be right up his alley

I'm not that sure, since he does not want to do illegal stuff (like for the belgian test track where the factory is now closed to the public). As for visiting those parts legally, well, I'm only aware of a few instances where there has been the media granted access. But it was more for documentaries about how the police works down there, not for a general history lesson. Both the police nor the IGC (the french administration managing the quarries) will bother with Tim sadly. So yes it'd be up his alley if he could do it legally.

> They’ve been cracking up on trespassers recently

It's actually the opposite. There's a constant cat-and-mouse game going on, and they're switching strategies recently, which I could experience first-hand. Current strategy is about prevention because they know they cannot prevent people from getting down there, so they're emphasizing safety and not hurting yourself because they "don't want to get woken up at 9AM a sunday morning because someone got lost".

> Tim-like pedantry corner: the quarries and the catacumbs are actually the same thing. Catacumbs are parts of the quarries that have been turned to ossuaries, and are reachable from the wider quarry network.

Fair enough. It’s still useful to distinguish them because they evoke different things. Also, aren’t the bits we can visit physically separated from the broader quarry network due to cave-ins?

> I'm not that sure, since he does not want to do illegal stuff

He certainly used to do this sort of things (and I did not check recently but in the past he posted some videos filmed in places where he was not allowed to be).

There are places that are normally closed off but that can be open occasionally like during the heritage days. At least one quarry, the Montsouris reservoir, and I think parts of the old sewers as well. Those are already quite cool and interesting.

> It's actually the opposite. There's a constant cat-and-mouse game going on, and they're switching strategies recently, which I could experience first-hand.

There’s always a bit of both. I don’t have a recent first hand experience, it’s just a mate who works with the Parisian fire brigade. There are near misses regularly, though rarely as bad as the teenagers who got lost for 3 days a couple of years ago.

> they're emphasizing safety and not hurting yourself because they "don't want to get woken up at 9AM a sunday morning because someone got lost".

Makes sense. Going on a rescue mission before the morning coffee is just not done :)

> Also, aren’t the bits we can visit physically separated from the broader quarry network due to cave-ins?

The bits that can be visited legally is the museum, and it's isolated with man-made walls, not accidental cave-ins. It's very much on purpose. There have been several occurences of people digging holes from the illegal part to the museum for fun (like doing parties), swiftly fixed by the IGC. Speaking of the IGC, their sole purpose is to avoid said cave-ins.

And to expand a bit about the pedantic distinction: there are multiple ossuaries in the largest network of quarries: the official one, and two others only accessible illegally. There's one under the Montparnasse cemetery (the one visible in most catacumbs youtube videos), and another one near porte d'orléans.

> At least one quarry, the Montsouris reservoir, and I think parts of the old sewers as well. Those are already quite cool and interesting.

Yup! Those are definitely worth a visit. And fun fact, there are two "layers" to the montsouris reservoir: the top one with the water, that can be visited, and the bottom ones, which are the foundations of said water "tank". It's a forest of large pillars with a few artifacts. AFAIK those are not visitable legally, but were illegally due to a crawlspace between the illegal part of the quarries and it. It was quite fun :)

> There are near misses regularly

Indeed. Most of the times where the fire brigade is called is when there are accidents in the accesses to the quarries: opening manholes, falling off ladders, etc. And whenever such an accident occurs, the authorities closes off the access to avoid further accidents. Well, unless it's a safe one to avoid more dangerous ones to be opened by people. I've used several such accesses that where ... not smart to use, to say the least :)

(oh and pass the bonjour to your mate from someone I hope he'll never have to rescue!)

> teenagers who got lost for 3 days a couple of years ago.

this is very rare; usually from a combination of drug use and sheer bad luck. given how many people there are dwelling during the weekend, it's rare for noone to stuble on you when lost.