diff --git a/crates/skate-data/src/retail_collision.rs b/crates/skate-data/src/retail_collision.rs
index 9fd651da..de5e7bc8 100644
--- a/crates/skate-data/src/retail_collision.rs
+++ b/crates/skate-data/src/retail_collision.rs
@@ -9,6 +9,8 @@ pub struct RetailTriangle {
     pub points: [[f32; 3]; 3],
     pub edges: Option<[u8; 3]>,
     pub surface: u16,
+    /// Unit flag 0x80: the unit carries a surface ID (`surface` is 0 otherwise).
+    pub has_surface: bool,
     pub group: u16,
     pub one_sided: bool,
 }
@@ -38,8 +40,16 @@ fn id(data: &[u8], at: &mut usize, width: u8) -> Result<u16, String> {
     Ok(value)
 }
 
-/// Visits complete clusters in archive order, retaining only one decoded
-/// cluster at a time. The callback can build its own spatial query index.
+/// Visits complete clusters in archive order, retaining one decoded mesh at a
+/// time. The callback can build its own spatial query index. Returns the number
+/// of triangles visited.
+///
+/// Meshes in which no unit carries a surface ID are skipped. Across every
+/// shipped district these are only 12-triangle boxes (zone/trigger/bounds
+/// volumes); authored player positions lie inside some of them, e.g. the
+/// SkateSchool area and DownTown's Kube Tower landing, so they are not solid.
+/// Meshes that mix surfaced and surfaceless units are ordinary geometry and
+/// are kept whole.
 pub fn visit_clusters(
     data: &[u8],
     mut visit: impl FnMut(&str, &[RetailTriangle]) -> Result<(), String>,
@@ -96,6 +106,7 @@ fn mesh_clusters(
     let one_sided = be16(data, 60)? & 0x10 != 0;
     let mut previous_end = table + count * 4;
     let mut total = 0;
+    let mut clusters = Vec::with_capacity(count);
     for i in 0..count {
         let offset = be32(data, table + i * 4)? as usize;
         if offset < previous_end {
@@ -109,11 +120,17 @@ fn mesh_clusters(
         previous_end = offset + size;
         let triangles = decode_cluster(cluster, granularity, data[62], data[63], one_sided)?;
         total += triangles.len();
-        visit(&triangles)?;
+        clusters.push(triangles);
     }
     if total != be32(data, 40)? as usize {
         return Err("RWCM triangle count mismatch".into());
     }
+    if !clusters.iter().flatten().any(|t| t.has_surface) {
+        return Ok(0); // volume, not collision (see visit_clusters)
+    }
+    for triangles in &clusters {
+        visit(triangles)?;
+    }
     Ok(total)
 }
 
@@ -192,6 +209,7 @@ fn decode_cluster(
             points,
             edges,
             surface,
+            has_surface: flags & 0x80 != 0,
             group,
             one_sided,
         });
diff --git a/crates/skate-data/tests/retail_collision.rs b/crates/skate-data/tests/retail_collision.rs
index f98192c1..d7f98954 100644
--- a/crates/skate-data/tests/retail_collision.rs
+++ b/crates/skate-data/tests/retail_collision.rs
@@ -64,6 +64,44 @@ fn preserves_all_vertex_encodings_and_little_endian_unit_ids() {
     assert_eq!(clusters, 3);
 }
 
+/// Removes the surface ID from the first unit of fixture mesh `index`: clears
+/// unit flag 0x80 and shortens the unit stream by the 2-byte ID (its last field).
+fn strip_surface(bytes: &mut [u8], index: usize) {
+    let be16 = |b: &[u8], at: usize| u16::from_be_bytes(b[at..at + 2].try_into().unwrap());
+    let be32 = |b: &[u8], at: usize| u32::from_be_bytes(b[at..at + 4].try_into().unwrap()) as usize;
+    let mut at = 12;
+    let mut mesh = 0;
+    for _ in 0..=index {
+        let name_len = u32::from_le_bytes(bytes[at..at + 4].try_into().unwrap()) as usize;
+        at += 4 + name_len;
+        let size = u32::from_le_bytes(bytes[at..at + 4].try_into().unwrap()) as usize;
+        mesh = at + 4;
+        at = mesh + size;
+    }
+    assert_eq!(bytes[mesh + 63], 2, "fixture surface IDs are 2 bytes");
+    let cluster = mesh + be32(bytes, mesh + be32(bytes, mesh + 52));
+    let unit_start = (usize::from(be16(bytes, cluster + 4)) + 1) * 16;
+    assert_ne!(bytes[cluster + unit_start] & 0x80, 0);
+    bytes[cluster + unit_start] &= !0x80;
+    let length = be16(bytes, cluster + 2) - 2;
+    bytes[cluster + 2..cluster + 4].copy_from_slice(&length.to_be_bytes());
+}
+
+#[test]
+fn skips_meshes_without_any_surface_ids() {
+    let mut bytes = FIXTURE.to_vec();
+    strip_surface(&mut bytes, 1);
+    let mut names = Vec::new();
+    let total = visit_clusters(&bytes, |name, triangles| {
+        assert!(triangles.iter().all(|t| t.has_surface));
+        names.push(name.to_owned());
+        Ok(())
+    })
+    .unwrap();
+    assert_eq!(names, ["compression-0", "compression-2"]);
+    assert_eq!(total, 2);
+}
+
 #[test]
 fn rejects_every_truncated_prefix_and_trailing_bytes() {
     for end in 0..FIXTURE.len() {
diff --git a/docs/hails-additions/05-collision-volumes.md b/docs/hails-additions/05-collision-volumes.md
new file mode 100644
index 00000000..0164a86a
--- /dev/null
+++ b/docs/hails-additions/05-collision-volumes.md
@@ -0,0 +1,99 @@
+# 5. Invisible collision volumes
+
+## Problem
+
+On SkateSchool the player hit an invisible wall in the middle of the spawn
+area and could not skate around. Logs showed normal physics states (no
+failures), so the obstruction was in the collision data.
+
+## Root cause
+
+Retail collision archives contain a small number of clustered meshes in which
+**no unit carries a surface ID** (unit flag `0x80` unset; units read as flags
+`0x21` instead of the usual `0xa1`). Every one of them, in every shipped
+district, is a 12-triangle axis-aligned box: zone, trigger or level-bounds
+volumes. The game's loader (`skate_world::retail_collision_world`, via
+`skate_data::retail_collision::visit_clusters`) turned every triangle into
+solid collision, so these volumes became invisible walls, floors and ceilings.
+
+SkateSchool has seven, all in its global stream (`cSim_Global.xsf`):
+
+| Box (X / Y / Z, metres) | Effect in game |
+|---|---|
+| −79…16.5 / −1.53…6.65 / −43…52 | covers the whole school area; the authored start (y 0.11) is inside it |
+| −66…−62 / −1.53…6.65 / −3.6…12.7 | the invisible wall in the spawn area |
+| −11…2.5 / 0…4.16 / −2.6…1.6 (three identical) | invisible block near the origin |
+| level bounds, Y −73…79 | floor under the whole map |
+| one more box | — |
+
+Evidence that these are not solid in the original game: every authored
+SkateSchool locator (Coach Frank, the hub, all tutorial stations) is at
+y 0.0–0.4 on the real floor (surface 258), i.e. inside the big box; and in
+DownTown, a surfaceless box (X 259–333, Y 48.9–57.7, Z −152…−71) encloses
+the Kube Tower landing zone (`Z_DT_CubeTower`, y 55.9) and four authored
+challenge start points.
+
+Before the spawn change in doc 4, SkateSchool's spawn (y 7.63) stood on top
+of the big box, which hid the problem.
+
+## Survey of all districts
+
+Classified per mesh by surface-ID presence:
+
+| District | Fully surfaceless meshes | Notes |
+|---|---|---|
+| SkateSchool | 7 (all 12-triangle boxes) | global stream |
+| MaloofMoneyCup | 2 (12-triangle boxes) | global stream |
+| DownTown | 2 (12-triangle boxes) | city cell streams |
+| MegaPark | 1 (12-triangle box) | global stream |
+| others | 0 | |
+
+**Mixed** meshes (some units with surface IDs, some without) are common in
+real geometry: University cells (up to 31k triangles), Industrial, DownTown
+(138 meshes), BlackBoxPark, and the main geometry of several parks. A
+per-triangle filter would therefore remove real floors; the rule must be per
+mesh.
+
+## Change
+
+`crates/skate-data/src/retail_collision.rs`:
+
+- `RetailTriangle` gains `has_surface` (unit flag `0x80`).
+- `mesh_clusters` decodes and validates a whole mesh (including the triangle
+  count check) before visiting its clusters, and skips the mesh when no unit
+  has a surface ID. `visit_clusters` returns the number of triangles visited.
+  Memory: one decoded mesh at a time instead of one cluster.
+
+The fix is in the game, so existing installations benefit without an asset
+refresh.
+
+## Verification
+
+- New test `skips_meshes_without_any_surface_ids` (fixture mesh with its
+  surface ID stripped is skipped; the others are visited intact). All
+  `skate-data` library and integration tests pass, as do the game's
+  collision/world tests.
+- Collision triangles loaded per map (`SKATE_RWCM_READY`), before → after:
+  SkateSchool 91,790 → 91,706 (−84 = 7 boxes), DownTown −24, MaloofMoneyCup
+  −24, MegaPark −12, all other maps unchanged. Every map passes
+  `--check-assets`.
+- In play: SkateSchool (spawn area free to skate) and DownTown tested
+  thoroughly; the Maloof and MegaPark box locations (below) were visited
+  afterwards and are also clear.
+
+Further evidence the volumes are triggers: MegaPark's only box
+(X 61.4–73.2, Y −4.1…4.1, Z −140.8…−132.6) surrounds the stadium's arrival
+point from the world (`tele_world_to_stadium_dest_locator_01`, 2 m from its
+centre), and one of Maloof's two boxes lies 14 m from its arrival point from
+DownTown (`tele_dwtn_to_mmcp_dest_locator_01`). The other Maloof box
+(2.8 × 3.4 × 2.0 m) is 17 m from a street challenge start.
+
+## Notes for upstream
+
+- `cargo test -p skate-data` currently fails to compile three examples
+  (`apt_data`, `hud_data`, `scoring_flow_data`) that reference game-only
+  modules; this predates this change. `--lib --tests` runs the test suite.
+- The mesh-level rule is empirical (all 12 such meshes in the shipped data are
+  boxes, and authored player positions lie inside some). If retail code that
+  consumes these volumes (zones/triggers) is ported later, they should be
+  routed there instead of dropped.
diff --git a/docs/hails-additions/README.md b/docs/hails-additions/README.md
index d9bd2a20..643563d2 100644
--- a/docs/hails-additions/README.md
+++ b/docs/hails-additions/README.md
@@ -12,7 +12,8 @@ Each document stands alone. File paths are relative to the repository root.
 | 1 | [ISO extraction argument order](01-iso-extraction.md) | Setup (Python) | Done, verified with a real ISO |
 | 2 | [Windows long paths during setup](02-long-paths.md) | Setup (Python) | Done; UAC path not exercised end to end |
 | 3 | [SDL3 gamepad input](03-sdl3-gamepad-input.md) | Game (Rust), build scripts, vendored `sdl3-sys` | Done, verified in play with an Xbox Elite Series 2 |
-| 4 | [Authored map spawns](04-map-spawns.md) | Asset pipeline (Python) | Done; in-game confirmation in progress |
+| 4 | [Authored map spawns](04-map-spawns.md) | Asset pipeline (Python) | Done, all 10 maps confirmed in play |
+| 5 | [Invisible collision volumes](05-collision-volumes.md) | Game data loader (Rust) | Done, all four affected maps confirmed in play |
 
 ## Environment used for verification