diff --git a/docs/formats/core-data-regions.md b/docs/formats/core-data-regions.md
index 3491bde..a870785 100644
--- a/docs/formats/core-data-regions.md
+++ b/docs/formats/core-data-regions.md
@@ -242,3 +242,29 @@ index, rather than with a fixed-layout record.
 **So the 368 stride stands and the records remain unidentified.** The shrub
 method failed here not because it was the wrong tool but because it needs
 something constant to latch onto, and there is nothing.
+
+## Sky opens with a 256-byte-row image
+
+**Range measured:** level 0, `2192576 .. 2615232` (the sky region, 422,656
+bytes). Periodicity start and end located by sliding an 8 KB window in 256-byte
+steps through the region and measuring the 256-stride match rate.
+
+**Method:** autocorrelation as above, then a periodicity scan. A random
+control of the same size reads flat at 0.0000 for every stride tested.
+
+**Result:**
+
+- Peak stride in the first 64 KB is **256** at a 0.772 match rate, against a
+  flat random control. Strides 255, 257 and 4 score nearly as high, which is
+  what a true period looks like smeared by sampling.
+- Across levels the peak is 256 on 21, 512 on 5 and 128 on 1. 512 and 128 are
+  a harmonic and a sub-multiple, so the fundamental period is **256 bytes**.
+- The periodicity holds from `+0` to about `+60160` and then stops: **236 rows
+  of 256 bytes**, i.e. a 256-wide, 236-tall image at one byte per pixel.
+- The remaining ~362,240 bytes of the sky region are *not* 256-periodic and
+  remain unidentified. That is 86% of the region, and it is where the offset
+  table seen at `+0x20` earlier points.
+
+So the sky region is not one thing. Its first ~60 KB is an image; the rest is
+whatever the layer offsets address. Both halves still need work, and the
+image is the tractable one.