diff --git a/airdef-mod/src/sim/mod.rs b/airdef-mod/src/sim/mod.rs
index 7cb7b3b..b2d005a 100644
--- a/airdef-mod/src/sim/mod.rs
+++ b/airdef-mod/src/sim/mod.rs
@@ -509,7 +509,13 @@ impl Ground {
     /// Explosions change the land: forget what was known round them.
     fn forget(&mut self, at: DVec3, radius: i32) {
         let (cx, cz) = (at.x.floor() as i32, at.z.floor() as i32);
-        self.known.retain(|&(x, z), _| (x - cx).abs() > radius || (z - cz).abs() > radius);
+        // A blast affects a small square. Removing those columns directly avoids scanning the
+        // entire height cache once per explosion (and again when its confirmation times out).
+        for x in cx.saturating_sub(radius)..=cx.saturating_add(radius) {
+            for z in cz.saturating_sub(radius)..=cz.saturating_add(radius) {
+                self.known.remove(&(x, z));
+            }
+        }
         self.world.touched(at, f64::from(radius) + 2.0);
     }
 }
diff --git a/airdef-mod/src/sim/regress.rs b/airdef-mod/src/sim/regress.rs
index b8b4b11..3dc9c48 100644
--- a/airdef-mod/src/sim/regress.rs
+++ b/airdef-mod/src/sim/regress.rs
@@ -1989,3 +1989,20 @@ fn cold_ejection_power_and_coast_use_their_own_speed_rules() {
     assert!((fly(spec.cold + kind.burn_ticks(), 2.0) - 2.0 * 0.995).abs() < 1e-9, "coast drag");
     assert!((fly(0, 0.2) - 0.4).abs() < 1e-9, "cold ejection minimum");
 }
+
+#[test]
+fn blast_forget_removes_only_columns_in_the_same_square() {
+    let mut ground = Ground::default();
+    for x in -25..=25 {
+        for z in -25..=25 {
+            ground.known.insert((x, z), (x * 100 + z, 123));
+        }
+    }
+    let before = ground.known.clone();
+    let at = DVec3::new(-3.2, 70.0, 4.9);
+    ground.forget(at, 8);
+    let expected: std::collections::HashMap<_, _> = before.into_iter()
+        .filter(|&((x, z), _)| (x + 4_i32).abs() > 8 || (z - 4_i32).abs() > 8)
+        .collect();
+    assert_eq!(ground.known, expected);
+}