In a broad sense, the whole of the spirit world- paradise and hell- is a “spirit prison,” inasmuch as the spirits there, even the righteous, look upon the long absence of their spirits from their bodies as a bondage (see D&C 45:17; D&C 138:50; see also D&C 138:15, 16, 18). “I know it is a startling idea,” President Brigham Young stated, “to say that the Prophet [Joseph Smith] and the persecutor of the Prophet, all go to prison together… . But they have not got their bodies yet, consequently they are in prison.” (JD 3:95; compare Moses 7:55-57.)
It is in this sense that Christ went and preached to the spirits in prison (see 1 Peter 3:18-20; 1 Peter 4:6). President Joseph E. Smith beheld in vision that the Savior went not in person to preach to the wicked in the postmortal spirit world, but rather that he organized his missionary force in that realm and commissioned the righteous to preach the gospel to the spirits who were in darkness and under the bondage of sin (see D&C 138:20-22, 29-32). Christ went to the spirits in prison in the sense that he went to the spirit world.