The entrances to the Coral Roads are all hidden, and each is unique. There was a Kalen's Journal intro in one of the Atlases describing a Navigator opening a doorway for them. Here's the description from the MA3Ed with some description:
These beautiful, ancient submarine highways were constructed
(or grown) in the Interregnum between the First and Second Eras
by a mysterious group known only as the Earthwardens. Aboveground
entries to the network are cleverly hidden in rocky coastlines
or lonely atolls. Over the millennia some of these entryways?
and some tunnels themselves?have been flooded or sealed off by
the naturally growing coral. The Coral Roads usually follow island
chains and undersea ridges, always in shallow water. Inside,
the roads are arched corridors made up of coral and shell; some
areas are translucent to allow filtered sunlight to illuminate the
tunnels by day. Phosphorescent corals provide a dim blue-green
light at night. Periodically along the route and at junctions there
are airlocks with doors made of huge bivalve shells. The floors are
uniformly smooth and flat, like mother-of-pearl.
It is believed that the Earthwardens built the Coral Roads as a
secret, secure way to travel about the planet without being seen
by powerful rivals?possibly surviving K?ta?viiri. It is also assumed
that they used some sort of vehicles or beasts of burden to traverse
the roads swiftly; perhaps precursors of the Tartu.
There is no known existing complete map of the Coral Road
system, and the few partial maps are closely guarded.
