The Bible speaks of this era as being a time of great wickedness. There were giants in the earth in those days; some translations call them the Nephilim. These giants were the offspring of the "sons of God", usually interpreted as angels, who descended from the heavens to beget them with mortal women. These giants or Nephilim were unusually powerful; Genesis calls them "heroes of old, men of renown;" but also says that they were an evil race, whom God repented of creating, and therefore He sent the Flood to wipe them all out, all except Noah and his family. Nevertheless, the Nephilim reappear much later in the Biblical narrative, in Numbers 13:31-33.
These giants, and the Flood, were once considered to be the source of large fossil bones, and the explanation of how seashells and other traces of marine life came to be found in rocks on mountaintops. Evidence of large extinct forms of life were thought to have been the remnants of large creatures exterminated together with the race of giants.
"Antediluvian" is sometimes used figuratively to refer to anything of great age, and in this capacity is a common "SAT word"