The operation was to rig down the mouse hole in preparation to nipple down 21.1/4” BOP. The mouse hole was lifted in vertical position by crane using wire rope sling. While lowering the mouse hole to the ground, the sling slacked as the mouse hole touched the ground. One sling eye rotated enabling the safety latch to open and the eye came out of the hook. The mouse hole (650 kg) dropped to the ground from a height of around 6m. No injuries occurred.