Southwest of Hilo in the Hawaii Volcanoes National Park on the Big Island, the Napau Crater Trail (aka Mauna Ulu Trail) is an adventurous route to the Puu Oo Vent, which is the center of the current eruption. This 18 mile round trip hike will require all day.
The trailhead is off Chain Of Craters Road between milepost 3 and 4.
After about a half hour hike from the trailhead, there is a side trail leading up Puu Huluhulu, the summit of which offers a nice vista of Mauna Ulu. Beyond this side trail, the route becomes more difficult and the footing less certain. The route heads to the massive Makaopuhi Crater and then to the Napau Crater at the 7 mile point. From here, its only another 2 miles to Puu Oo, and this is the most scenic part of the trail. The trail heads near the base of the volcano so watch out for which way the wind is blowing to avoid volcanic fumes.
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.
Overall, Napau Crater Trail is the 9th most popular hiking trail of all 45 hikes in Hawaii.