Marble is extremely heavy and, despite feeling solid, cracks easily under its own weight if carried flat. The rule with any stone slab is to always keep it vertical, on edge, from the moment you lift it.
Pro tip: Move marble on edge like a large pane of glass, and support it against a padded vertical surface at all times.
Watch out: Never lay a marble slab flat to carry or transport it; unsupported in the middle it will crack under its own weight.