Hell isn't the grave.
Revelation 6:8
And I looked, and behold a pale horse: and his name that sat on him was Death, and Hell followed with him. And power was given unto them over the fourth part of the earth, to kill with sword, and with hunger, and with death, and with the beasts of the earth.
Revelation 20:14
And death and hell were cast into the lake of fire. This is the second death.
The spirits of Death and Hell will be cast into eternal torment in the Lake of Fire and will be tormented for ever and ever along with all those men whose names were not found in The Lamb's Book of Life.
Revelation 14:9-11
And the third angel followed them, saying with a loud voice, If any man worship the beast and his image, and receive his mark in his forehead, or in his hand, the same shall drink of the wine of the wrath of God, which is poured out without mixture into the cup of his indignation; and he shall be tormented with fire and brimstone in the presence of the holy angels, and in the presence of the Lamb: and the smoke of their torment ascendeth up for ever and ever: and they have no rest day nor night, who worship the beast and his image, and whosoever receiveth the mark of his name.