Thunder drums are a type of drum built and used by the Philiraki giants in western Hellea. An ancient practice of the giants, the drums function primarily as methods for long distance communication. When struck they can be heard for tens of miles (though are usually used on shorter scales), and are almost deafening up close. Messages are sent by converting words to complex sequences of drum beats - as each sequence corresponds to a word instead of a letter or sound, the full vocabulary is very large and takes a great deal of time to learn. The exact details of how to construct these drums are a closely guarded secret, which the drum masters only teach to carefully chosen apprentices.