Pellucidar is one of the many creations of Edgar Rice Burroughs.  While most people associate him with only the creation of Tarzan, he also dealt with a fictional version of Mars, called Barsoom in the John Carter series of novels, among others.  Pellucidar is a land inside the earth and was the basis for the film "At The Earth's Core".  The following pages are hopefully a brief introduction and the "E-Book" is accessible here and on other sites (see below).  I will try to provide both a "printer-friendly" and a downloadable version here soon.