BSN: HADES/SHEOL

This was the place of the dead (aka the netherworld, the underworld) in biblical times. It was called “Hades” in the New Testament (from the Greek) and “Sheol” in the Old Testament (from the Hebrew). Until the time of Jesus, everyone who died went below (a form of sleep); it was Jesus who re-routed death so that it leads above to heaven; for a full explanation of how Jesus re-wired death so that it led up instead of down, see the book The Biblical Case for Everyone Going to Heaven. As for when this happened, see the book The Biblical Case for the Second Coming as Accomplished Fact.
The word “Sheol” appears 65 times in the Old Testament; and it appears another 10 times in the New Testament as the Greek word “Hades.”
“Hell” is something entirely different.
“Abaddon” is a related but more obscure term.