The Tunnels and Caves

Overview

The network of caves and tunnels beneath the town — the creatures' daytime lair, the site of Martin's dungeon, the home of the music box, and the burial chamber where the seven children were sacrificed. Tabitha Matthews fell into them when the Matthews' basement hole collapsed (S1E10), and they have been the town's most important forbidden geography ever since: the answer to the mystery lies below.

Description & Rules

The creatures' lair. The monsters sleep here by day (S1E10). Victor Kavanaugh tells Tabitha "the underground is where the creatures sleep" — entering during daylight means walking through a sleeping army.
Extent. The network is vast and strange. Victor and Tabitha explore it in S2E01 and find what appears to be a forest underground, narrowly escaping when the creatures begin to wake. The mineshaft where Boyd Stevens lands after his faraway tree trip (S1E10) connects to the same system, delivering him to the dungeon.
The dungeon. A stone chamber where the emaciated Martin was chained to a wall (S2E01–02). The music box stood here; it played as Boyd watched. Martin died after transferring his blood-worms to Boyd.
The burial chamber. The place where the town's first residents sacrificed their seven children (S3E10 origin reveal). Jade Herrera finds the tunnel where the sacrifice happened (S4E05) and the children's bones (S4E09), pursued by creatures as he and Tabitha gather them.
The ruins. Stone ruins in the forest mark the dungeon entrance; Julie Matthews story-walks here (S3E07, S4E02), and the ruins are where Boyd returns in S2E10 to destroy the music box.
The symbol. Jade's first vision of the seven children shows them chanting at an opening in the ceiling that matches the Symbol — the tunnel ceiling carries the pictograph (S2E10).
Access points. The Matthews' basement hole (S1E10), the ruins, the mineshaft, and — critically — the roots of the bottle tree, which connect directly to the burial chamber (S4E09–10).

Appearances / History

S1E10 — Tabitha falls into the caves; the creatures' daytime lair is revealed.
S2E01 — Victor and Tabitha explore; they find an underground forest and flee waking creatures.
S2E02 — Boyd, having climbed out of the mineshaft, meets Martin chained in the dungeon; the music box plays; Martin dies.
S2E10 — Boyd returns through the ruins, sees the three cicada victims chained in the dungeon, and destroys the music box. Jade's vision of the seven children at the ceiling opening.
S3E06 — Victor and Henry Kavanaugh find Jasper's ventriloquist doll in the tunnels and flee waking creatures.
S4E05 — Jade finds the sacrifice tunnel.
S4E09 — Jade and Tabitha find the children's bones in the chamber as creatures close in.
S4E10 — After the bottle tree is uprooted, Jade and Tabitha flee through the caves and become trapped; Boyd, Ellis, and the transformed Fatima rescue them.

Connections

The Monsters' Behavior — the creatures sleep here by day.
The Music Box — stood in the dungeon; destroyed there.
Martin — chained in the dungeon; "tip of the spear."
The Seven Children — the burial chamber is the site of their sacrifice.
The Bottle Tree — its roots open into the chamber.
The Symbol — carved on the tunnel ceiling.
Faraway Trees — the mineshaft connects Boyd's tree-trip to the network.
The Talismans — found in a cave in the woods (S1E08 flashback), possibly part of the same system.

Theories & Open Questions

How far does the network extend — and is the "underground forest" a separate pocket of the town, or a route to somewhere else entirely?
Was the dungeon built for Martin, or was he imprisoned there by the same force that chained the three cicada victims in S2E10?
Is the burial chamber the origin of the town's power — the place where the immortality bargain was struck?
What else sleeps below that the creatures protect? Martin's "tip of the spear" warning implies something worse.
What lies beneath the bottle tree roots — the last unexplored depth of the chamber — now that the tree is gone?