Southwest of Hilo off Saddle Road on the Big Island, there is a spectacular lava tube, known as Emesine Cave, that is off the beaten path. It is a rarely used cave from the 1881 lava flow that is in great condition. This is not a well marked or well used route. Inside, there are lava stalactites, shelves and channels.
All lava tubes are potentially dangerous. They may seem stable, but you will see areas of rock collapses. Also, while the cave may be very tall, it does neck down in spots and you can easily hit your head. A hard hat is recommended. A strong light is highly recommend -- preferrably a divers light, not just the camping headbands.
To reach the trailhead from the Kona/Waimea area, head east on Saddle Road (SR200) to milemarker 22. Continue to about 0.4 miles east of the milemarker 22. Look for a rough trailhead parking area on the right. While it looks like a parking area, there is a small 4WD road (aka Kulani Powerline Road) beyond a gate leaving the parking area. The 4WD road used to service an old power line route that no longer exists. Park here.
From the parking area, start hiking south on the flat Kulani Powerline Road for 2.5 miles. Expect the weather to be misty or raining. After passing through a lush fern grove, the road descents down to a different lava bed. Roughly 200 feet after the descent, the lava tube opens on the right. After entering the tube, follow the tube back under the road. The tube runs for about a mile, with at least one skylight opening and no side routes. The tube ends at a collapse and it is not recommended to continue any further. Return back through the tube to the road. For an interesting return route, you can pickup the Puu Oo Trail which leads back to the Saddle Road about a half mile west of the parking area.
Note: a state permit is required to hike this route. Contact the state at (808) 587-0166. You may be then told that no permit is required, depending upon who you get on the phone.
Overall, Emesine Cave is the 19th most popular hiking trail of all 45 hikes in Hawaii.