The video above shows the underground chamber of the Nekyomanteion, aka the Necromanteion, the "Oracle of the Dead," which the ancient Greeks believed was the door to Hades -- the Greek underworld, not the Hell of Christianity.
"The Necromanteion ... was an ancient Greek temple of necromancy devoted to Hades and [his wife, the goddess of he underworld] Persephone," according to Wikipedia. "According to tradition, it was located on the banks of the Acheron river in Epirus, near the ancient city of Ephyra. The site was believed by devotees to be the door to Hades, the realm of the dead."
"The Greeks used several terms for oracles of the dead. Nekyomanteion, 'prophecy-place of the dead' is found first, in the 5th century BC." Source: "The Ancient Greek Orackles Of The Dead," Daniel Ogden (slow-loading PDF).
"Although there were many oracles in ancient Greece, there is only one known oracle of the dead (1)," according to the Atlas Obscura. "It was an entryway to the underworld, and a place where the shades of the dead could - under special circumstances - reveal the future to the living. It was the place that Homer sent Odysseus to seek advice from a long-dead oracle, and where he was horrified by visions of Hades, and it also appears in a sordid chapter of Herodotus' Histories."
"Communicating with the dead, however, was not limited to the famous characters in the writings of the ancient Greek authors," according to Ancient Origins. "Regular citizens [also visited] the Necromanteion to seek counsel with the dead. They would first enter a dark chamber before performing certain elaborate rituals intended for their own protection and to gain the ability to communicate with the deceased. After this, a priest would lead them into a deeper chamber, where a ritual animal sacrifice would be performed, before passing through three gates which symbolised their entry into the Underworld. The celebrants would now be able to speak to the spirits of the Underworld."
(1) The Atlas Obscura is incorrect when it says that there was only one Greek oracle of the dead. According to the Ogden paper cited above, there were four major oracles: "Acheron in Thesprotia [the oracle discussed here--named after the river], Avernus in Campania [Italy], Heracleia Pontica on the south coast of the Black Sea, and Tainaron at the tip of the Peloponnese's Mani Peninsula."
Note: As mentioned in the footnote above, the oracle on the Acheron river was located in Thesprotia. According to the Oxford Classical Dictionary (3rd ed. revised), the Thesproti were "a people mentioned in Homer's Odyssey with a king of Ephyra in the Archeron valley." They formed "a tribal state which entered the Epirote Alliance and the Epirote Confederacy, sided with Rome in 170 BC, and survived under the Roman settlement."