Were the Chronicles of Narnia books based on something like the Hollow earth theory or?

was it supposed to be an entirely different planet all together?

The Chronicles of Narnia books were based on the bible.

thought it was a christianity thing...

It's supposed to be a different planet. In The Magician's Nephew we see Aslan, who is presented as how Jesus appears in that world, singing Narnia to life and calling forth the sun and stars for that world. In other words, it's the creation of THAT universe, as opposed to ours. As a matter of fact, Narnia is a flat world -- in fantasy, you can do anything! -- and in Voyage of the Dawn Treader they reach the "edge" of the ocean where it drops off like water pouring over the edges of a giant table.The author C.S. Lewis had Christianity in mind as the basis behind Narnia, but he assumes that God can create any sort of universe -- and in fact has created an infinite number -- so some of the worlds in those other universes have wonderful creatures and things in them that are only "imaginary" in our world.

C.S. Lewis was a born-again Christian and the entire set was a Christian alegory. Read it in that context and you can't miss the symbols thrown in your face.

The book is based on stories of the bible.The Lion is representative of Jesus and the Crucifixion as well as the Resurrection.The evil brother can be viewed as Judas.The sisters can be viewed and Mary Magdalen.The white witch as the Devil.I read that the author of the book was a very religious man and wanted to tell stories of the bible in a child's mythical view.

Narnia is based off of C.S. Lewis's opinions and ideas on Christianity, fully blown into a set of tales written to express his ideas in a story format. It was also his way of getting into competition and thumbing his nose at Tolkien, who was once his closest friend and then became his rival, as Tolkien was a Catholic and Lewis was a Protestant.But since much Christian allegory draws from many of the same archetypes as other religions, you can see similarities in other ancient religions as well.