This is a new publication of James Jeans' famous book The Mysterious Universe. Despite that there have been advancements, mostly in particle physics and cosmology, which occurred after the publication of this book, it is still one of the masterful presentations of the main ideas of the two major revolutions in fundamental physics in the twentieth century - relativity and quantum mechanics - and their implications for our understanding of the Universe.Perhaps Jeans' most provocative suggestion is: "Many would hold that, from the broad philosophical standpoint, the outstanding achievement of twentieth-century physics is not the theory of relativity with its welding together of space and time, or the theory of quanta with its present apparent negation of the laws of causation, or the dissection of the atom with the resultant discovery that things are not what they seem; it is the general recognition that we are not yet in contact with ultimate reality."From the cover of the 1937 Pelican Books publication: "The Mysterious Universe... at once achieved a tremendous popularity and broke all records for a serious scientific work. It has since been translated into many languages, and is famous throughout the whole of the civilized world."