I am not an engineer of earthworks, so would appreciate constructive criticism (pun intended)
I am thinking of a scenario where an old Covenant once built an emergency escape tunnel to a farm they secretly controlled, perhaps 1~2 miles away, on the other side of a wooded hills. Tunnel made by a young mage with strong Terram, but minimal experience with tunnels.
Tunnel still survives a century later, but in poor shape. Based on the landscape, it was 10~50 feet underground. Presumably no ventilation (nor lights)
What other hazards would one expect in such a tunnel?
If it's an old tunnel that has never been used or checked, forget about lighting, ventilation and other engineering hazards, make it mythical. Consider magical moles, the root of the local magic tree, earth elementals, faerie dwarves, gnomes and goblins that somehow found their way in.
The tunnel now leads to a network of Regios and other ethereal realms.
Kids get lost and come out Elsewhere.
Time travellers wander in and out.
A nest of Demons lives in part of the system.
One idea I've seen in fiction & real life is sink holes can develop in a tunnel network, so sometimes people, animals and other things fall in. Maybe something wants to lure and catch people into that.
Maybe the farm on the other end is now a Castle, and the magi are on the wrong end of someone else's escape route.
Looking at dangers in mines, some type of bad air or airborne dust seems to be a common one. You could have sections suffering effects similar to that generated by the spells 'Stench of the Twenty Corpses', 'Wreaths of Foul Smoke', 'Fog of Confusion', or 'Room of Stale Air' (all Auram from Core). These could be natural rather than magic so MR would not protect.
Hold on. In Neil Gaiman's "Neverwhere", wasn't the Great Beast of London originally a hog that escaped the butchers into the sewers, and fed well? This tunnel goes to a farm...
For another twist on cave gases, consider methane. Your trigger-happy Flambeau (or any grog carrying torches) is going to have a bad surprise.
The tunnel might connect to a network of underground caves (either on purpose or by accident). Then you have a whole micro-system for the players to explore.
Wet and/or uneven ground can present some challenge to ill-prepared magi and grogs (had a grog botching an athletics roll once and cracking his skull when falling, good times).
Did the young Terram magus leave any traps behind? Either mundane or magical? Consider a charged device enchanted with a modified version of Pit of the Gaping Earth. Parma won't protect you from a 40 feet fall. OTOH, maybe simply a mundane sinkhole.
A lot depends on climate and where the tunnel was dug (or carved- is it in porous earth or carved into bedrock? If the soil has been absorbing moisture (even intermittently) then it is likely collapsing as this will have eroded the structural strength, while if it is made of stone it lacks drainage and is likely flooded, possibly with eels or other things living in the water. Remember also in mythic Europe still air is bad air, so the air itself will be toxic without any kind of air flow, and the same applies to standing water. At 50 feet down it won't be likely to freeze unless one end or the other is very open to the elements (generally a frost line will be less than 2 feet of depth), but if there is airflow to prevent toxic air then it might well result in frozen water- at least periodically.
There is also a strong probability of fungal growth from either whatever might have been left down there to decay or from the soil itself depending on what portion of it was effectively partially composted before being compressed (by the dirt above)
The tunnel is supposed to go under a wooded hill. If the woods are oak, then the fungus can include truffles in the oak roots.
Which might feed a feral pig...
whose rooting for truffles might expose a little ventilation, but not enough for entry/exit.
Oak roots go down 18 inches, well short of 50 feet. 50 feet down you won't be finding any root under an oak forest, just compacted soil or rock depending on what kind of geological structure underpins the hill.
Also the lifespan of a feral pig is about 15-20 years, and they start having children at 6 months, so after a century being closed up you would be dealing with the inbred 50th to 200th generation of the original pig's descendants- at which point cannibalism is likely a portion of their diet as well as fungal growths. In the real world the energy potential would have been long depleted in that time frame and they would have died out, but in Mythic Europe they could well survive but would likely have been overcome by infernalism between incest and cannibalism, even as dumb animals.
If there is any ore in the tunnel that was originally formed by the terram magic you could also be dealing with the oxidization of the metalic content of the ore rocks as well, results could vary depending on the type of ore as well as how oxidization might work differently in mythic europe versus the real world.
Also again recall that oxygen diffusion through soil is not a real thing in Mythic Europe, which depends on circulation of air rather than oxygen. If someone left an enchanted chamber of spring breezes effect down there the air is fresh, if it has been still, it is bad.
