Southwest of Hilo in the Hawaii Volcanoes National Park on the Big Island, the Ka-hau-a-lea Trail is an challenging 8.4 mile round-trip route to a view point of the Pu u Oo volcanic vent. The route is actually through the State of Hawaii Kahaualea Natural Area Reserve, not the National Park (who will tell you this is not a legal trail because they are jealous). It was build in 1990 to access the vent. It passes through the most scenic and most lush rainforest on the island. This route is a very difficult jungle mud slog over rolling, rocky, rooty, muddy terrain through lush ohia-hapuu forest. It will take at least 6 hours round-trip and it is highly recommended to start around 7am so that you are back well before dark in case of problems. Plan on at least 4 hours each way. Make sure to stay on the trail since the jungle is quite thick and you will most likely become disoriented and loose the trail (yes, this has happened often). While there isnt much elevation change, the trail rolls along through the forest. Watch out for dangerous cracks in the bedrock on the trail.
The trail ends at the forest edge with a view of the Puu Oo lava cone. The forest edge was created by lava flows and volcanic gases destroying the forest. Do not continue past the edge of the forest for your own safety. The lava actually flows down the opposite side. Acces to the slopes and cone are strictly prohibited by the National Park Service. Matter of fact, the National Park Service will even deny that this trail exists!
To reach the trailhead from Hilo, head southwest on SR11 for 20 miles. Between milepost 19 and 20, turn left heading south on South Glenwood Road. Veer right and the left. The road changes its name to Captains Drive / Ala Kapena. The road eventually becomes narrow and unpaved at about 2.5 miles from the highway. Continue for 1 more mile to the dead end and the trailhead. In May 2006, the trailhead was signed.
Here are some notes on adventuring in Hawaii. First, unlike mainland forests, the tropical island forests can grow very dense and can mask dropoffs. Plus, the trails can be very wet and slick most of the year (December to February is the main rainy season, but expect rain all year round). Stay on the trail to avoid disorientation. Second, speaking of disorientation, be aware that most Hawaiian topographic maps use the Old Hawaiian Datum. When trying to use the latitude/longitude from those maps with a GPS unit, you will think you are 0.4 miles south/southeast of where you really are. Kind of important in a dense jungle. Third, the bacteria Leptospirosis is found in the local waters and mud. It causes flu like symptoms within 2-20 days of contact, and can be fatal if not treated with antibiotics. Fourth, when hiking about, walking sticks can be very helpful for stablity in the slick conditions, for probing mud hole depths and clearing webs of the tiny crab spider.
Overall, Kahaualea Trail is the 17th most popular hiking trail of all 45 hikes in Hawaii.