The Bible speaks of Sheol and Hades as the abode of the dead. It is located in the “lowest part of the Earth” according to many quotes from the Scriptures. It is characterized as “the bottomless pit”.
In Numbers chapter 16, we read the sad story of Korah. He and other prominent men (250 princes in total) rebelled against Moses’ authority. They wanted to occupy the position God had given Moses and Aaron.
In the end, God punished Korah, the men with him, and their families by making them “go down alive into the pit”.
And the earth opened her mouth, and swallowed them up, and their houses, and all the men that appertained unto Korah, and all their goods.
They, and all that appertained to them, went down alive into the pit, and the earth closed upon them: and they perished from among the congregation.  Num. 16:32-33
This is reiterated many times in the Scriptures. In Revelation we see the opening of the bottomless pit. So, it is safe to say that Hell or Hades is located in the center of the Earth, but this is a spiritual realm and as such, it can be located any place God puts it. The bottom line is that Hell is real, and all the wicked, who know not the Lord Jesus Christ will end up there (or down there).