Written in 1913, this was Ed's sixth book since his maiden effort, Under the Moons Of Mars (title later changed to A Princess of Mars). Even more incredible, this was the first novel of the THIRD extraordinary series Ed invented in his first two years of professional writing - Mars and Tarzan being the first two.
Whether Ed was familiar with the work of Jules Verne (Journey to the Center of the Earth) or Arthur Conan Doyle (The Lost World) is unsure. His primeval world under the Earth's crust may share ancestry with those two works, but the work could also be considered an amalgamation of themes from Barsoom and Tarzan!
In any event, Pellucidar takes its own unique place in the Burroughs canon - with cavemen, Stone Age mammals and dinosaurs aplenty for an adventure starved audience.
And it wouldn't be a work by Edgar Rice Burroughs without food for serious thought. Note the very interesting treatment of "time" in this world without time.