Wikipedia has more on Taphephobia
Taphephobia, subterraneapremortephobia or taphophobia (from Greek taphe, tapho, "grave") is the fear of being buried alive. This fear could've originated from stories they have heard in the past about the body decaying and being eaten by worms and bacteria. Some fear cemeteries (coimetrophobia) because they also fear seeing ghosts and other monsters. This fear could've been brought about by a personal experience being scared in a cemetery, or even just watching horror movies.
Taphephobia also applies to the fear of being enclosed or being buried alive and is true for those who work underground such as miners. They fear the suffocation they know that will happen should they be buried alive.
These people try to avoid going to the cemetery, are scared of watching horror movies, and would sometimes even avoid going to wakes because just seeing a coffin triggers the fears. They will go great lengths to avoid going underground, but if they do they manifest with signs and symptoms of anxiety such as palpitations, shortness of breath, trembling, feeling of running away even when they only think of the things mentioned above.
Treatment includes psychotherapy and medications may be prescribed if symptoms are severe enough.