Sub Terra

Readiness

draft-ready (confidence=0.74, rules=0.65, fun=0.90). BGG rank: 791; year: 2017; weight: 2.13; playtime: 60 min

Core Loop

Sub Terra is a cooperative survival game where 1–6 players are cavers trying to escape an underground cave system before running out of oxygen. The cave is explored by flipping tiles from a shuffled deck, expanding the map as players move into unexplored areas. On each turn, a player takes 2 actions (move 1 tile, explore an adjacent tile, help an ally, or use a special ability). After all players act, the Hazard Phase activates: draw a Hazard card, which may cause flooding (place water tokens on tiles — players on flooded tiles lose health), spawn gas (instantaneous damage to players in that area), cause a collapse (seal a tile permanently), or advance the oxygen/panic track.

Players lose oxygen each round. If the Oxygen track depletes, remaining oxygen loss starts killing players. The only win condition is finding and exiting via the Exit tile (face-down until discovered in the exploration) before everyone dies. The Exit tile is shuffled into the bottom third of the tile deck, so players know roughly when it might appear.

Turn Structure and State

• No manual/BGA/transcript source is present; card relies on memory plus BGG context.
• BGG description anchor: Description from the publisher: A 1-6 player cooperative game of terrifying cave escape. Players take the role of amateur cavers attempting to escape an unexplored network of subterranean tunnels, before the lights flicker out or the darker things beneath the Earth catch up to them... In Sub Terra players spend their turn exploring and revealing the tunnel system around them, attempting to survive the various [...]

Win Condition and Arc

All players who make it to the Exit tile and then off the board win together. Players eliminated (health reduced to 0) do not survive. Other survivors still win. The game ends either when remaining players escape or when the last player dies. The arc is an urgent exploration sprint: the first half is mapping the cave quickly to find the exit; the second half is often a frantic retreat toward a known exit while the Hazard Phase escalates (flooding spreads, oxygen depletes).

Decision Primitives

BGG mechanisms: Action Points, Cooperative Game, Dice Rolling, Grid Movement, Map Addition, Modular Board, Point to Point Movement, Push Your Luck, Tile Placement, Variable Player Powers

Memory-derived primitives:

• Cooperative tile exploration (flip tiles to expand cave map)
• 2-action economy per player
• Hazard card draw per round (flooding, gas, collapse, oxygen loss)
• Oxygen track (shared depletion = game clock)
• Health track per player (flooding and gas damage)
• Exit tile in bottom third of deck (partial location knowledge)
• Specialist roles (each player has a unique ability)

v4 controlled primitives: variable_player_powers, escalating_threat, event_deck_seeded_threat, partial_observability, cascading_failure

Top iOS archetype fits: coop 5.4, balatro 4.8, snap 3.8.

Why It Is Fun

Sub Terra creates claustrophobic tension through accumulated board state — flooded tiles form walls of danger, collapsed tiles permanently close routes, and gas pockets lurk as instant-damage traps. The oxygen clock creates constant urgency without a fixed turn limit, making every "safe" turn feel like a gamble. The variety of hazard types means the threat is unpredictable (you don't know if the next card floods, collapses, or spawns gas), and the cooperative planning required to coordinate 2-action turns in a multi-branch cave is genuinely fun.

Player-voice evidence:

• Awesome Game. Really cool to play with a UV light. A bit on the shallow side, but a fun theme.
• The game plays well and give a nice sense of exploration. The main drawback is that tiles are too dark and it's hard to see what's going on in them.
• Co-op tile placement game where players must work together to escape the depths before running out of time or being captured by the Horrors. Our game came down to the final two dice rolls to escape, but we lost. So close! ? You must stay...
• Fun, easy to play, replayable, suspens ... and not expensive
• Modular co-op. Elegance of carcassone mixed with the fun of betrayal type exploration. Very difficult but extremely fun and tense. Only negative so far is that some roles you’ll be relying on a lot and some have specific abilities you...

Friction and Failure Modes

• Treat Sonnet-memory edge rules as draft until confirmed by manual, BGA, or transcript.
• Wikipedia source is flagged as a possible title mismatch; do not use it as evidence.
• Needs at least one stronger rules authority before final extraction use.

Translation and Design Hooks

• Use this card to ask: which primitive carries the fun if theme/licensing is removed?
• For iOS, look for short-session compression, clear state visualization, and a digital-only twist.
• For new tabletop design, look for the tension source and decide whether to preserve or invert it.

Edge Rules and Gotchas

• Players who become Exhausted (health = 0) don't die immediately — they collapse and must be rescued by another player (adjacent, spend 1 action to help them up to 1 health), but if they take any further damage while Exhausted they're eliminated
• Flooding: tiles can hold 1 water token; if a second flood card would affect the same tile, it spreads to adjacent tiles following specific rules
• Collapsed tiles are removed from the map and cannot be entered — this can isolate players or cut off exit routes permanently
• Each specialist role has 1–2 unique actions: the Climber can traverse collapsed tiles; the Medic can heal adjacent players; the Geologist can look at upcoming tiles
• The panic track (separate from oxygen in some editions) escalates when players die, increasing hazard frequency

Sources Used

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    "kind": "bgg_comments",
    "path": "data/bgg_comments/204472.txt",
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    "note": "positive/player-voice sample"
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  {
    "kind": "llm_memory",
    "path": "data/llm_memory_sonnet/204472.md",
    "quality": 0.65,
    "note": "sonnet-self-rated-7"
  },
  {
    "kind": "tabletopia_overview",
    "path": "data/tabletopia_overviews/204472.md",
    "quality": 0.3,
    "note": "Tabletopia overview; not a rules authority"
  },
  {
    "kind": "wikipedia",
    "path": "data/wikipedia/204472.md",
    "quality": 0.15,
    "note": "possible-title-mismatch: Inside the Box Board Games"
  },
  {
    "kind": "llm_memory_opus",
    "path": "data/llm_memory_opus/204472.md",
    "quality": 0.1,
    "note": "sonnet-self-rated-3-unknown"
  }
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