diff --git a/docs/DESIGN.md b/docs/DESIGN.md
index c1615e5..5703815 100644
--- a/docs/DESIGN.md
+++ b/docs/DESIGN.md
@@ -521,6 +521,12 @@ above, including:
   invisible, undrillable basalt the game treated as built. Now any melt except metal and glass
   cools into natural rock wherever it came from, and any solid no block, mold or machine owns can
   be drilled, so nothing can get permanently stuck in the world.
+- **Resonance pylon: metal sets hardness, power sets reach (t47, 2026-10-27).** A pylon is built
+  in copper, iron or steel, which caps the hardest rock it can break (4, 5, 6), the same way
+  bits and tanks step up through metals. Power from the shaft network sets its radius and pulse
+  rate, up to a cap per metal; power never sets hardness, so it can't skip the steel bit. It is
+  tuned like the drill and never harms anything built. It waits until power is switched back on,
+  since today every machine runs without power.
 
 ## Ideas not built yet