HONOLULU (KHNL) - It's Hawaii's most famous natural landmark, over a half million people hike to the top of Diamond Head every year.
But while many take in the scenic sights, there are many parts of this popular park that are off limits to the public.
For some, its not the easiest of hikes to the top of this landmark. A nearly mile long trek from Diamond Head Crater to the summit, going up 560 feet, and many keep an eye on the rough trail.
But the next time you head to this scenic spot, keep this in mind and take a look around inside the crater.
"When you're walking up the trail you're walking into history, said Yara Lamadrid-Rose, Diamond Head Park Coordinator.
The trail itself, carved out of this old volcanic tuff cone was created over a hundred years ago, as the military built fortifications from Pearl Harbor to Diamond Head.
Many of the old pillboxes remain, dotting the summit's landscape. While this empty field used to be the site of shooting ranges.
And tunnels can be seen along the crater walls, off-limits to the public.
Inside, there are no windows in these offices, and you can see how solid the original bunker was - which was built to fire massive artillery mortars.
"The walls were made of 9 feet thick concrete with a very strong roof to protect munitions and protect people from bombardment," said Ed Teixeira, State Civil Defense.
It's the only bomb proof offices in the state, but working here inside the mountainside is not for everyone.
"To work in a place like this with no windows it seems like you are underground - it takes a certain personality," said Texeira.
Some of these old sites at diamond head are being used again, while others remain restricted. But they are all visible reminders of diamond head's role to defend oahu from attack.
Even with all the defenses, no artillery was ever fired from diamond head during war.
And in the future, the state hopes to open up more of the park to the public as the military clears out of the crater.