I know there were portals discovered in the Phrygian city of Hierapolis, with the traditional Greek symbolism associated with the underworld. I'm curious as to whether there are anymore Greek portals, or sites from other cultures associated with an underworld.
In the Greek world the main two would be at Lake Avernus in central Italy, next door to Cumae; and Lake Acherousia in Epeiros (now dried up). There were suggestions floating around that the rivers that ran through or near these places were linked in the Underworld: viz. the Phlegeton at Cumae, and the Acheron and the Kokytos at Acherousia. Both were known as nekyomanteia, that is, "oracles of the dead", where people could go to consult dead spirits via local priests known as psychagogoi ("soul-summoners").
Odysseus' fictional consultation of the prophet Teiresias, in the Homeric Odyssey and in a lost Aeschylean play, is usually considered to have been set at one or other of these places; and it's usually reckoned that the story of Hades kidnapping Persephone and taking her down to the Underworld was set at Acherousia, though that's not directly attested. And there are various other mythical overtones to both places.
At Cumae there was additionally a cave in the side of a hill (still there) that involved going underground. It's suspected there may have been a cave associated with Acherousia too, but no ancient source explicitly attests it. Other sites also sometimes involved tortuous clambering through underground spaces to consult the oracle: that was also the case at the oracle of Trophonios in Boiotia, for example.
Other than those, there were also oracles of the dead at Heracleia Pontica, on the north coast of Turkey; and at Tainaron, on the southern tip of the Peloponnese. (Another mythical resonance: the last is supposedly where Herakles brought Kerberos up from the Underworld.) For more reading I recommend Daniel Ogden's book Greek and Roman Necromancy (2001).
The Mayans appear to have viewed cenotes (sinkholes of a certain type) as gateways to the afterlife, or at least an underground world worthy of offerings.
http://archive.archaeology.org/0405/abstracts/maya.html
www.mesoweb.com/pari/publications/journal/504/Cenote.pdf
I can't find a great citation, but some Aztec sites had underground passages linking various burial areas which were too small for humans and unlikely to be infrastructural. http://phys.org/news/2011-05-tunnel-temple-mexico.html
There's also the Hittite reverence for springs, caves and underground watercourses as passages into the underworld. Hittite texts very regularly speak of the underworld as a cthonic realm(the "Dark Earth") associated with the subjugation of previous gods and the concealment of evil, but also with fertility, the sun, and even kingship*. The Hittites revered entrances to this realm and listed them as divinities alongside other geographic features and even as recipients of offerings and witnesses to treaties, under the name dKASKAL.KUR, 'path of the land'.
Where this becomes of interest to you is that we have at least one example of a manmade 'entrance to the underworld', the Sudberg or 'sacred pools' at Hattuša. This was a massive ritual installation containing a trench and two underground chambers which seem to have been partially flooded as part of a large sacred pool comparable to other sacred pools-in other words, a man-made ritual version of one of these sacred springs. One of these chambers in fact has an inscription in Hieroglyphic Luwian ending with the phrase "I constructed a divine earth-road", along with a relief on the back wall of a sungod and next to it a relief of the king Suppiluliuma II deified. So this chamber was likely a symbolic entrance to the 'dark earth', and probably used ritually in the same way that natural springs were as a liminal space(it's suggestive that the iconography of the site seems to relate, like that of the Hittite monument of Yazilikaya, to the deieification of Hittite kings) and suggests that other Hittite pools like the installation surrounding the spring at Eflatun Pinar(with its reliefs of a sun goddess and storm god supported by mountain gods) were understood as similar cultic installations.
*In the Kumarbi cycle it is where the defeated gods go, but it is also associated in other texts with the great patroness of many Hittite kings and protector of the land the Sungoddess of Arinna/Wurušemu, with the afterlife journey of the Hittite kings on their way to deification. Gods could in Hittite thought move in and out of the Netherworld; thus for example the appearance of the "Vanishing god" myth, in which a god goes "into the dark earth" and must be placated. EDIT: Fixed a minor copyediting thing
Only from my travels to Nicaragua every year have I heard of the local lore of Volcán Masaya just outside the capital (Managua). It's locally referred as "La Boca del Infierno" the mouth of hell. There is a cross at the highest peak of the mouth of the volcano. I've seen a special on it but can't remember which network it was on. Weather it was NatGeo, Discovery, or History channel. People would commit suicide by jumping into the mouth which still puts out sulfur on most days. I'm on mobile right now, but when I get to a PC I'll provide links.
Not sure if this is really what you're after but it's a Greek site associated with the underworld ... http://www.tripadvisor.co.uk/ShowUserReviews-g189452-d2706042-r160714057-Acheron_River-Parga_Preveza_Region_Epirus.html ... this river is known as "the Styx" and their is a cave mouth that you can walk along the gorge too that is known as Hades. I'm not sure how long it's been known in that way whether it was just a recent invention for the sake of attracting tourists I just happen to have visited and waded in the very frigid waters one Summer.
