The Black Fang (mini?)heroquest provides an entrance under the walls of
the Rubble ("This way +, this way -, this way Humakt") It also comments that
the Pavis temple also uses this way for questing, although it goes
somewhere else on the heroplane when they use it (this makes the methods of
questing even more interesting if you can use one of the "normal" entrances
but end up in different areas depending on, for example, the ritual that's
used).
Another entrance, from Sun County (? - do I remember rightly), is in the Old
Yelmalio Temple where there's a link to a staircase in Yelm's mansion in
Heaven.
Robert McArthur