Absolutely awesome!  Volcanic activity is categorised from levels 1 to 5.  Up to level 3 it is possible to visit. Above 3 it's considered too dangerous. Island evacuated at 5. We visited at '2.5'. Eruptions were occuring every few minutes, often throwing huge (some as big as an articulated trucks) glowing rocks & lava bombs hundreds of feet into the air above our heads. Each eruption began with an incredible 'whooshing' noise, followed by billowing mushroom clouds of sulphur laden smoke, & then the ground shook and........KAA-BOOOM!  We 'felt' the noise/shockwaves rather than simply hearing them.  Behind & around us was littered with previously ejected lava bombs/rocks, although they most commonly fall back into the crater. Nevertheless it would have been most unwise to turn your back on the crater! Most that failed to fall back into the crater landed within 100 metres of where we sat, one however had me pretty worried for a bit though. We were there for 2 or 3 hours & experienced 40 or 50 eruptions. Scary but fantastic. The 'rim' is not solid rock, but rather volcanic ash (like corse black sand) & we were concerned that at any time we could be part of a 'landslide' down into the crater, but Peter, our guide seemed pretty comfortable. No safety features like guard rails etc. Each time we were feeling anxious a look at Peter's composure was our only reassurance.. Just getting there & back was an adventure in itself, 4wd'ing up incredibly steep, washed out muddy & rocky jungle tracks. And then there was the walk back down the sides of the volcano, slipping & sliding in pitch darkness with only a tiny torch to guide our way back to the vehicle over ash, rocks & occasional steaming fissures with the ongoing explosions behind us, after which the same vehicular tracks were once again negotiated. Brilliant!