The existence of charge density wave (CDW) correlations in cuprate superconductors has now been established. However, the nature of the CDW ground state has remained uncertain because disorder and the presence of superconductivity typically limit the CDW correlation lengths to only a dozen unit cells or less. Here we explore the field induced three dimensional (3D) CDW correlations in extremely pure detwinned crystals of YBa2Cu3Ox (YBCO) ortho-II and ortho-VIII at magnetic fields in excess of the resistive upper critical field (Hc2) where superconductivity is heavily suppressed. We observe that the 3D CDW is unidirectional and possesses a long in-plane correlation length as well as significant correlations between neighboring CuO2 planes. It is significant that we observe only a single sharply defined transition at a critical field proportional to Hc2, given that the field range used in this investigation overlaps with other high field experiments including quantum oscillation measurements. The correlation volume is at least 2 3 orders of magnitude larger than that of the zero-field CDW. This is by far the largest CDW correlation volume observed in any cuprate crystal and so is presumably representative of the high-field ground-state of an ideal disorder-free cuprate. Show Partial Abstract