This doesn't really count as exploring I guess, but it's what I did this weekend and the previous one (before that I just did the hike).
Anyway.. it's here on O'ahu, over near the southeast corner.
Basically it's a bike (17km each way) from my place in Manoa through Waialae-Kahala, over to Hawai'i Kai and then up Koko Crater which gives a fantastic view of that part of the island.
The ride itself is fairly uneventful (minus a small hill climb at the end and lots of headwind as you head east against the tradewinds), and the hike is hot and dusty, but the view is gorgeous.
The hike itself starts at a public park, and you walk through some tall grass to get to the trailhead. The trail is the remnant of a tram railway that goes up the side of the crater where there used to be a radar station to guide coastal batteries (apparently).
It's pretty dry and dusty, and frankly not that exciting. It's basically just sheer will grinding up what starts as like maybe a 15-25% grade for the first half and then after you cross a rail bridge it turns into more of a 30-50% grade. The rail bridge can be a little iffy the first time, especially coming down because it's somewhat steep, and a few of the ties are either missing or semi-rotted. The bees buzzing around and the prospect of falling through the gaps for 20 feet or so to the gully below doesn't help either. There are a few spots where if you fell you would quite literally go down which isn't a big deal except for the first time I went up and I got a little dehydrated and sorta teetered on the edge a bit. It's not a big deal for someone with even moderate backcountry experience.
You're basically hiking up the tramway so there's rails, an old cable, as well as electrical utility lines (now defunct, but army-installed so they'll be there for another hundred years - we want to take a sawzall and chop them down but they're pretty tough). This does make it a lot easier than if it were just loose gravel (Hawai'i has 'rotten' rock) and mud, because the ties are pretty good at providing traction. However they're sufficiently decrepit that it's not just like climbing up stairs.
When you finally get to the top it's all worth it. The heat, the dust, the misery, it all melts away as you're greeted with a beautiful view down into the crater itself (You've ascended the leeward side which is higher because when it erupted more ash was deposited downwind) as well as all the way to Diamond Head, back into the valley behind Hawai'i Kai. Haunauma Bay is especially pretty. The wind rushes up the 'bowl' of the crater and is amplified to some degree creating a really incredible blast that cools you off instantly. The windward side of the crater is remarkably steep because of the extra weathering it gets. There are some beautiful rock formations created by the wind on the rim of the crater itself. The rim is hikeable though I have not done it (yet.)
There are a network of blockhouses and tunnels which I have not explored yet (gotta get a camera first). Some have been welded shut, but others (like a 20+ ft deep open latrine) are rather dangerous. There are also some nifty cacti as well as one which produces a 6-petaled flower.( Pictures and maps behind cutCollapse )
Well.. hopefully that interested someone, and I hope more people join this community! I love to hear about what other people do.