From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
The Eightfold Furnance (八卦炉, Hakkaro; FUNimation "Magic Furnace") is located within Mount Gogyo.
Although the exact connection to Yama remains unknown, Eightfold Furnace was a doorway that connected Earth and the Next World. Once past a door, the visitor had to overcome several mindtricks and illusions of attackers. The Eightfold Furnace was in the middle of Mount Gogyo and guarded by Annin. According to Grandpa Son Gohan, people from the living world could come here on special occasions and visit their deceased ancestors. The steam from the Eightfold Furnace was what allowed both the dead and the living to travel safely, and if the fire stopped even for a moment it would take over 200 years to re-ignite without the Basho Fan, giving enough time for dark spirits to take over the living world.
Because the Eightfold Furnace was very old, holes at the bottom eventually formed and caused a fire to emerge on the other side of the world from underground and consumed Gyumao's Castle on Mt. Frypan. Turtle Hermit eventually put out the fires around Ox King's Castle with the Original Kamehameha, but several years later they reignited. Son Goku and Son Chichi came to the Eightfold Furnace in a final desperate effort to save Ox King, and Goku eventually repaired the holes at the bottom. This location is neither seen nor mentioned again.
This is the first and only place in the Next World to appear in the Dragon Ball anime. The Eightfold Furnace only appears in the final anime filler episodes of Dragon Ball leading up to Dragon Ball Z. Ox King's Castle still erupted in flames early in the Dragon Ball manga, but the cause was never revealed and the flames were presumably extinguished by Turtle Hermit.