Complex sentence: In the 1970s, Clark embraced some paranormal ideas to explain UFOs and other unusual phenomena; he was influenced by the "ultraterrestrials" theory of John Keel, and the so-called interdimensional hypothesis (which had been championed by Dr. Jacques Vallee).
Split into simpler sentences: In the 1970s, Clark embraced some paranormal ideas to explain UFOs and other unusual phenomena. He was influenced by the "ultraterrestrials" theory of John Keel, and the so-called interdimensional hypothesis (which had been championed by Dr. Jacques Vallee).