Deepwater drilling today – in the Gulf of Mexico and other basins worldwide – often involves drilling both salt and subsalt intervals. Over the past decade, the use of expandable concentric underreamers or "hole openers" in deepwater drilling has become widespread. An estimated 75% of the salt/subsalt sections in the GoM now use underreamers. There are various reasons to enlarge a hole, including to enhance equivalent circulating density, to reduce cementing pressures, and to counter salt creep so casing will reach bottom. Underreamers are especially important in deepwater wells that require numerous casing strings.