What is the first instance of a portal to another world?

In ancient Greece, the gods lived up on a mountain, and Hades seems to have been a place that you could sail to.

Personally, I think in modern times the concept of a portal or wormhole is so familiar, we tend to retroactively assign its presence even when not described. Obviously, you can't just walk to Mount Olympus, so you must pass through some kind of transition from the real to the supernatural. That transition is some kind of gateway that, even if left undescribed by the people writing the original myths, we assume to be something like a portal.

It doesn't seem to me, though, that there is evidence that any of the writers of myths were thinking of portals like we do. It seemed more that Olympus was hard to find, but you could just walk there if given the right path. Their concept of what was unreachable was based more on unexplored lands beyond their maps than it was based on a notion of planes of existence.

Even Dorothy was blown by a Tornado to Oz, so maybe it's a place far over the horizon? Sure, she teleported back with magic shoes, but that's different from a portal, like the back of a closet into Narnia, or a special track at a train station. Teleportation is often portrayed as an instant transportation within the same plane of existence.

It must be fairly recent, but, what exactly is the first use in fiction of an explicit portal between two different realities? Where a protagonist in a story could look at a gateway and see an opening to a different reality. Whether by magic or by science.