I certainly wouldn't risk my expensive drone to a pointless experiment, but I nonetheless seek to know all the quirks and workings of my Mavic 3. I'm curious how the system would respond to the following scenario:
A Mavic 3 flies a few hundred feet in one direction. Operator stops it, turns it around 180 degrees and drops it down low to enter a tunnel opening. There's still an RC connection and operator continues flying deeper into the tunnel for a while until it loses RC. What would happen next?
Thanks in advance for entertaining my curiousities...

Simple .. inside a tunnel, there is no GPS reception.
If your drone has no GPS reception and it loses signal, the drone lands where it is.

Hi there. Thank you for reaching out. The DJI Mavic 3 will automatically change to Attitude mode (ATTI) when the Vision Systems are unavailable or disabled and when the GNSS signal is weak or the compass experiences interference. In ATTI mode, the aircraft may be more easily affected by its surroundings. Environmental factors such as wind can result in horizontal shifting, which may present hazards, especially when flying in confined spaces. We recommend flying the aircraft in a safe area to avoid issues. Hope this helps.

DJI Paladin Posted at 3-14 23:31
Hi there. Thank you for reaching out. The DJI Mavic 3 will automatically change to Attitude mode (ATTI) when the Vision Systems are unavailable or disabled and when the GNSS signal is weak or the compass experiences interference. In ATTI mode, the aircraft may be more easily affected by its surroundings. Environmental factors such as wind can result in horizontal shifting, which may present hazards, especially when flying in confined spaces. We recommend flying the aircraft in a safe area to avoid issues. Hope this helps.

Why do the moderators here give such poor answers to simple questions about flying drones?

Labroides Posted at 3-14 23:45
Why do the moderators here give such poor answers to simple questions about flying drones?

Moderators must be politicians.

Virgil Quick Posted at 3-15 12:24
Moderators must be politicians.

That makes no sense.
You'll note the moderator said nothing at all about the drone landing when it has no control signal + no GPS signal, which is what the OP was asking about.

Moderators here don't fly drones or have drone experience.
Moderators here copy and paste without understanding the questions they attempt to answer.

Labroides Posted at 3-15 14:38
That makes no sense.
You'll note the moderator said nothing at all about the drone landing when it has no control signal + no GPS signal, which is what the OP was asking about.

Totally agree, to be an admin here there's only a few requirements. 1. Be familiar with the keyboard shortcuts "Ctrl+C" & "Ctrl+V". 2. Have no experience with any DJI product.  

change to Attitude mode (ATTI) when the Vision Systems are unavailable
How did this get added in? Nobody said anything about the vision system being unavailable. The rest of the reply goes on as if there's no vision sensors being used, which makes the entire answer irrelevant. In the US technically, once the drone loses GPS it is supposted to be landed immediately. 14 CFR 89.110(a)(2) says:

The person manipulating the flight controls of the unmanned aircraft system must land the unmanned aircraft as soon as practicable if the standard remote identification unmanned aircraft is no longer broadcasting the message elements of § 89.305

Then the question that comes up is: At what point is a tunnel considered as indoors? Since the FAA has no jurisdiction over indoor airspace, where is that line drawn? What a rabbit hole.

Labroides Posted at 3-14 22:40
Simple .. inside a tunnel, there is no GPS reception.
If your drone has no GPS reception and it loses signal, the drone lands where it is.

Makes perfect sense.

Labroides Posted at 3-14 23:45
Why do the moderators here give such poor answers to simple questions about flying drones?

Appreciate you pointing this out becuase I read the mods post and it actually just added more question marks than anything else.

Johnnokomis Posted at 3-15 19:22
Totally agree, to be an admin here there's only a few requirements. 1. Be familiar with the keyboard shortcuts "Ctrl+C" & "Ctrl+V". 2. Have no experience with any DJI product.  

change to Attitude mode (ATTI) when the Vision Systems are unavailable

Good info. Thx

Labroides Posted at 3-15 14:38
That makes no sense.
You'll note the moderator said nothing at all about the drone landing when it has no control signal + no GPS signal, which is what the OP was asking about.

I support! I am very surprised by the incompetence of administrators in all topics.