Edgar Rice Burroughs’ first Pellucidar book At the Earth’s Core was part of the supernova period at the beginning of his writing career, wherein he managed to write 25 novels between 1911-1915! The serialization of At the Earth’s Core in All-Story Weekly magazine in 1914 represents the extraordinary feat of launching three major literary franchises in a mere three years, following on the Mars/Barsoom series and the Tarzan series. Pellucidar's Hollow Earth setting with its weird timeless eternal day and its menagerie of threats from the chillingly alien Mahars, the brutish Sagoths, and various pre-historic megafauna remains one of the most sustained acts of invention in fantastic fiction to this day.

Although At the Earth’s Core was popular enough to be published in hardcover starting in 1922 and re-serialized in 1929, it doesn’t seem to have been in print after 1940. Certainly, David Innes is a likeable protagonist, but he lacks the larger-than-life qualities of John Carter of Mars or Tarzan of the Apes. Other suspects for At the Earth’s Core’s lapse into relative obscurity would be the World War II paper shortage, followed by Burroughs’ death in 1950.

At the Earth’s Core was first published in paperback by Ace Books in 1962, making it an early factor in the great Edgar Rice Burroughs revival of the 1960s. The lush and colorful cover by Roy Krenkel would certainly have helped it stand out on the racks:

The Frank Frazetta cover that graced At the Earth’s Core later Ace Books printings from the early 1970s through the 1980s depicts the horrible anticipation of the Mahar temple sequence.

The Pellucidar series is terrific worldbuilding but it did not leave as obvious an imprint on early Dungeons & Dragons as Burroughs’ Mars/Barsoom series, other than in its pulp ethos and sense of high adventure. The general pulp ethos was certainly present in Dave Cook and Tom Moldvay’s X1 - The Isle of Dread module which was included in 1981’s Dungeons & Dragons Expert Set. The Isle of Dread would pave the way for TSR’s Known World/Mystara setting and its undeniably pulpy/Burroughsian Hollow World sub-setting.