Advanced metalwork is unknown, as raw materials on the island are extremely rare. It has been observed, however, that they have made adroit use of metal objects which have washed up or been left behind on their shores, having some ability at cold smithing and sharpening iron and incorporating it into weapons and other items. For example, in the late 1980s two international container ships ran aground on the island's external coral reefs; the Sentinelese retrieved several items of iron from the vessels.[9]