They try translations with ideas from the illustrations as interpreted by Stoneheart.

Since nothing is directly attached to the passage it's illustrating there's still some pretty high uncertainty but- This one is probably 'magic', this one might be animal, that is either Dwellin or Person, and this group of characters that share a core ideogram are almost certainly different kinds of battles and fights or closely related nouns/concepts, and they can generate solid seeming guesses at structural words like 'yes' 'if' 'go' 'is' 'short(time)'. They figure out the number system too, base five with a zero character.

The core message on the door remains mysterious but sort of reads like a riddle, each 'sentence' starting with a number.

They haven't tried the door yet before a different archaeologist dispatches to the island.

There is definitely a large rectangular cavity down there. The first fifty feet of the access tunnel is filled in with dirt and rock, with grass growing over the outer entrance.

Way easier than the magic tunnel. Dig dig dig.

There's a small living area with long-rotten bedding and a little kitchen and no evidence of any use of fire, and a set of hallways into big stone rooms with rows and rows of sturdy stone shelves built right into the wall, which each have a short label and rows and rows of - stone tablets full of neat and tidy writing.

Oooooooh.

The archaeology team takes pictures of all the writing and tries to translate it.

Most of the recent stuff is in the current vernacular.

The older stuff has occasional Rosetta Stones, clearly specifically designed as translation aides. The oldest stuff somewhat resembles the writing in the ancient tunnel.

It looks like all the records are a sort of sparse history, with mentions of important empires and heroic figures, climate shifts and natural disasters, culture shifts, and the occasional philosophical musing on the nature of truth and eternity and change.

Wow, that's so useful! They send it to the tunnel team so they can translate it better and compare the records against what they can tell by other means about climate and empires and such.

It looks like these records are... Old. They don't mention specific dates, though they do seem to be in chronological order, but each shelf covers what seems like a full lifetime from a single writer and there are thousands. One of the more recent ones includes quite a lot of information about the Black Empire, the legendary aborted industrialization - they had steam power and industrial weaving and early mass production - along with lengthy distressed fretting about what went wrong, it seemed so promising!

The tunnel inscriptions seem to be describing the history of a desert empire whose key to power was the secrets of controlling magic animals. The stone library records have no mention of such an empire. And the hidden message on the door is an elaborate riddle about the properties of various animals.

Does the library have any solid guesses about what went wrong?

The tunnel archaeologists attempt to figure out the answers to the riddles, though they don't try to open the doors with them yet.

The librarian eventually decides that the gods wanted willing pawns and that they hate them, and that they were wasting space with rambling and everything they add has to be maintained, so the previous rambles have been massively edited down, and the rest of that shelf is incredibly dry. (The next shelf retains some anti-god bias, but it ebbs away forward in time, somewhat)

With that much greenpower on hand, they rapidly figure out the only plausible sequence of taps indicated by the riddle.

With considerable trepidation the magic grey does the taps. (A green has eaten the rocks they found in the mummy, but his power hasn't come in yet.)

At the final touch, the whole door emits a deep, low ringing sound sort of like a huge bell and - crumbles to dust that swirls in the air and vanishes in an expanding circle away from the grey, opening the way deeper into the tunnel.

They get it on video. The grey runs back up the tunnel as soon as it reacts, and then they cautiously advance as a group.

Just beyond there's the crumbling ruins of what looks like a bus station or something? No buses in evidence. The only sign says 'if you have passed (fortress?) you may use (unknown word related to magic?) to call transport animal'

...wow.

They will not attempt to call a transport animal at this time. They proceed, slowly, on foot, through the passage such an animal would take.

The 'bus' tunnel's roof is lined studded with things in the roof made of magic-rock-like material like the door, but otherwise look like an ordinary series of road tunnels, complete with sidewalks, what seems like ventilation shafts, and occasional side rooms with archaeologically interesting bits of stone that must have once had metal or wood attached to them. The place is worn but doesn't appear actually damaged.

Where do the ventilation shafts lead? Does the path branch? What seems to have happened to the metal and wood?

The ventilation shafts are hard to explore without robots. The path doesn't branch for the first couple of kilometers as it gently curves down and left, though the side rooms get steadily bigger and one looks like it was inhabited at some point.

There ought to be rust and maybe fossilized wood here, and dust and other faint detritus, but there's just - not? 

Huh. Maybe it was looted at some point. Though that would postulate that Dwellin were reading or that someone else was here... also that the door was replaced.

They set up a base camp, when they do get to a fork, and split off to explore the other tunnels.

The fork is a large cavern with six exits, bits of debris in the middle, and a large, faintly glowing structure like an upside-down crystalline tree in the roof. Not very bright, starlight levels at most, but certainly emitting light. The other five exits lead to:

An irregular pile of housing and workshops and assorted structures that collectively make up a sort of town that seems almost artfully messy rather than purely chaotic,
a series of large chambers that are lightless but have soil irrigated by slight dripping from degraded wall pipes and a small and weird ecosystem
a section labelled as a mine that quickly turns into a chaotic, cramped, and confusing to navigate warren of tunnels,
a grand hall with elaborate if faded mosaics and murals and six more grand doors blocking the way (this time without any writing on them),
a lengthy series of fortifications ranging from spike pits to tall stone walls to chokepoints with openings to shoot through blocking the way forward on the point opposite the initial entrance.

What kind of people lived here? Is it sized and marked like it was Dwellin, or someone else? What grew in the soil, what was taken from the mine? What are the artworks of? Is there anything more sophisticated than a spike pit in the way of traps?

The town definitely looks like it was inhabited by Dwellin! The soil is so old it's hard to tell but it looks like grasses and grains. The mine seems to indicate that it was for diamonds and silver, but... That doesn't actually make any geologic sense? The artwork is mostly about six abstract humanoid figures with distinctly non-Dwellin features doing things for, with, and to Dwellin. There are definitely more sophisticated traps. The poison gas has decayed. Several collapsing wall sections have let go a long time ago. The snakes falling from ceiling holes are mummies. The swinging blades get stuck when they trigger. The collapsing floors have all already collapsed. The - this was probably a bomb, but there's no trace of anything chemically active.

Do the abstract humanoids correspond obviously to the gods?