Travellers looking for an off-the-beaten-track destination in Japan might consider a boat trip to Tomogashima, which is the collective name for a group of four small islands called Okinoshima (the biggest island) Jinoshima, Torajima and Kamijima located off the small town of Kada in Wakayama Prefecture.

Shugendō Buddhist monks once used the steep hilly terrain of these islands as part of their ascetic mountain training.
This coastline was for centuries a haven for pirates who preyed on ships passing through the narrow entrance to the Inland Sea between Awaji Island and Honshu. One famous pirate was named Tsumujikaze Goemon and he was rumoured to have buried a stash of his loot on one of the islands which has never been found.
During the Meiji period the Japanese military fortified the islands with a number of brick-built gun batteries, powder magazines, bunkers and support facilities to defend the strategically important approach to Osaka Bay against foreign naval attack. Up until the end of World War II, access to the islands was strictly prohibited and their existence was removed from maps, hence the use of the word ‘hidden’. The gun emplacements are now overgrown and damaged by coastal erosion but you can see that they would have had a great view of any approaching invasion force.
Some say that the ruined defences bear a resemblance to those in the Studio Ghibli film Laputa, Castle in the Sky and may have inspired the artist. One of the ferries to the island is even called Laputa, obviously aiming to attract Ghibli fans.
Nowadays the islands, which form part of Seto-naikai National Park, are a popular place to visit for both Japanese and foreign tourists (mostly Chinese and Koreans). There are a number of well marked hiking trails around the island and in addition to the military remains there is a lighthouse, some quiet stony beaches, bbq and camping spots and lovely coastal views.
A noticeboard on the island says this about the lighthouse:
Viewing the beautiful scenery of the Seto Island Sea from the white, western style lighthouse is sure to lighten the heart of even the most downtrodden spirit.
There is some accommodation on the main island – Uminoie guest house.
Camping is free but you need to register at the office first.
We noticed a few warning signs about snakes and it was not long before we spotted a small one on the path which slithered away after a tense standoff (possibly a non-venomous rat snake?).

How to get to Tomogashima

A ferry service operates from Kada Port.
There are four sailings per day in each direction as you can see on this photo. Two additional sailings at 10am and 3pm during the peak holiday season.

The boat trip takes about 20 minutes.
The cost is JPY2000 return for adults and JPY 1000 for kids.

To get to Kada you can take a train from Wakayama City, Wakayamashi station.
Kada is famous for sea bream and there is a special pink coloured, fish-themed sightseeing train called Medetai at certain times of the day but our train was just an ordinary one.