Goa Jepang stored sad stories where Indonesian workers should work day and night with a very little food to fulfill Japanese ambitions during short colonization in Indonesia, to build this cave/tunnels. However if you just go into it without hear the story, things looked not so special. But if you take a guided tour, you will understand the reason, the role of this cave and all stories behind it. I did it in the second try when entering this cave. The total length is supposedly more than 1000 km, but the original plan, the total length of the tunnels should be more than 5000 km. I think the cave is still improved,because some historical part will be preserved e.g. room for ammunition. If you are afraid of long tunnel, narrow places and darkness, don't go inside. In short it is worth going inside when you are sure someone can tell the story, so having a guide is recommended. But, I was not sure when we could get guide who speak English, because when we were there, only Indonesian-speak guides were available. Some end of tunnels toward the valley are open, designed for hiding and escaping, and has a beautiful scenery.