Ancient Hawaiians used rocks from rivers to cook pig. They heated the rocks in the imu and the pig cooked with steam. They covered the pig with banana leaves, ti leaves, old mats and then dirt to keep the heat inside. This produced a unique taste that cannot be reproduced.
People today kalua pig in the same way that ancient Hawaiians did. Kalua pig is still served at luaus. However, it is illegal to sell kalua pig if it has been cooked underground. Hawaiians today still enjoy kalua pig that has been cooked in an imu at baby luaus and parties. Some companies try to imitate the flavor using liquid smoke to make kalua pig, but if you have had the real thing braddah, you can tell the difference!