Studies have shown that artificial reefs—which is what some offshore oil rigs have become —can be reconfigured to cultivate a limestone substrate on which coral can grow. Corrosion protection systems commonly used on offshore oil and gas facilities can cause growth of limestone on metal.  This was discovered more than 40 years ago and has been used for artificial reef projects around the world. Two of the largest such reefs are in Indonesia, on the northwest coast of Bali, and at Gili Trawangan on the northwest coast of Lombok.