Thanks Badger... and hey your idea of making it usable only by commanders with sailing is a great complement! IIRC, because I read that book when i was around 12yo, the characters have to do a difficult travel by a subterranean sea to get to the prehistorical land. So it is very thematic! The site is taking shape
What about your idea of the Japanesse underworld? What could you find in there?