1 Corinthians 15:18

Then they also which are fallen asleep in Christ are perished (KJV)

Then: Since the days of resurrection and ascension, the Corinthians had many friends who had already died. Those alive and those who had passed cultivated hope in Christ. Wright reminds readers that without the resurrection, those who have died only have a belief in the route of Hades available to them. i.e., the place where the dead cross the river Styx and stay in an abode of shadowy figures. (The Jewish word was Sheol.) Even here, there was no way out, for Jesus says in Revelation 1:18 that he has the keys of death and Hades (Miller and Miller, p. 240).

Now: We believe Jesus descended into the realm of the dead from I Peter (Jesus went to preach to the spirits in prison). Hades is all that would be left if Christ is not raised from the dead - for those who have already died and for those who are yet to die.

Extra Source: Harper's Bible Dictionary by Madeleine S. Miller and J. Lane Miller (New York: Harper and Row, Pub., c1961.