I was a candle

The Cumaean Canal is a fairly recent piece of engineering; a collaboration between Surface powers and the Masters. Almost definitely constructed during the Fifth City’s tenure. Legend has it that a passage to the underworld existed in Avernus, so there may have been an actual route down there beforehand; the engineers of the Canal used existing passages and caves to ease construction, but there may not have been one cohesive route down to the Neath. It’d be a hell of a trek, even if there was a way down.

The Travertine Spiral barely ever shows up so I have no idea when it was built at all.