Falling through a hole or trap door will cause damage

src/do.c

@@ -1302,6 +1302,8 @@ goto_level(
     struct monst *mtmp;
     char whynot[BUFSZ];
     char *annotation;
+    int dist = newlevel->dlevel - dunlev(&u.uz);
+    boolean do_fall_dmg = FALSE;
 
     if (dunlev(newlevel) > dunlevs_in_dungeon(newlevel))
         newlevel->dlevel = dunlevs_in_dungeon(newlevel);
@@ -1617,6 +1619,7 @@ goto_level(
             if (Punished)
                 ballfall();
             selftouch("Falling, you");
+            do_fall_dmg = TRUE;
         }
     }
 
@@ -1782,6 +1785,15 @@ goto_level(
 
     /* assume this will always return TRUE when changing level */
     (void) in_out_region(u.ux, u.uy);
+
+    /* fall damage? */
+    if (do_fall_dmg) {
+        int dmg = d(dist, 6);
+
+        dmg = Maybe_Half_Phys(dmg);
+        losehp(dmg, "falling down a mine shaft", KILLED_BY);
+    }
+
     (void) pickup(1);
 }