Plunging ranula under the chin of a 29 year old female patient. A ranula is a cyst caused by a blocked salivary gland, in this case the sublingual gland. If the cyst ruptures it becomes a plunging, or diving, ranula and the fluid leaks between the muscles of the neck. The cause of is often unknown, but can be due to trauma to the mouth. The ranula may disappear without treatment, but larger ranulas can need surgical removal of the cyst and the salivary gland to prevent reoccurance.