To the surprise of the test designers, who had expected a depression,it produced a permanent rubble mound called a "retarc" (an inverted crater - the word is "crater" spelled backwards). Permanent retarcs of this kind are restricted to hard rock where the shock displaced material, which has three times the volume of the original strata, is strong enough not to undergo compaction and subsidence. Strong rock like granite is especially conducive to this, since it tends to fracture in large blocks for maximum bulking effect. Sulky was in fact the only retarc producing event in the history of US nuclear testing. The mound was 24 m wide and 6 m high, and had a small depression in the center (9 x 3.5 m).