Imagination is undoubtedly the most difficult and the most particular passage means that would exist. It does not require apparently any particular place. The only force of imagination would be sufficient for some peopke to go physically on the obscure continent.
It is in any case what affirms Jules Verne. Celebrate writer of our world, it seems that Jules Verne is also a celebrity of the obscure world. It would have even made there published its first book that have been unaware for a so long time on our world: "Paris at the XXe century". Jules Verne affirms to have never used any passage or means of transport to go on the obscure continent. Only its imagination would have enabled him to go on this parallel world.
The case of Jules Verne seems unique in any case. No other testimony mentions passage by this means which seems very difficult to use.