Southwest of Hilo in the Hawaii Volcanoes National Park on the Big Island, the Halemaumau Trail is a 3.5 mile descent down 500 feet into the Kilauea Crater floor. It starts at the Halemaumau Overlook near the Visitor Center.
To reach the trailhead from Hilo, head south on SR11. Near milepost 29, keep an eye out for the entrance to the National Park on the left or south side of the road. After entering the park, stay to the right towards the Visitor Center and park there.
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, Halemaumau Trail is the 37th most popular hiking trail of all 45 hikes in Hawaii.