From "The New Key to Costa Rica":

Volcan Arenal is the quintessential volcano. Its perfectly conical shape emerges from Alajuela;s gentle green hills. From time to time, loud explosions are heard, a mushroom cloud of grey, brown, orange or blue smoke billows out of the top and you can see the sokey trails of ejected boulders as they bounce down the slopes.

Since Arenal was dormant until the late 1960's, the only people sho sus[pected that it was a volcano were those who had scaled it and found a crater and steam vents at the top. But few listened to them, until an earthquake shook the area on July 29, 1968. Twelve hours later, Arenal blew, sending out shock waves that were recorded as far away as Boulder, Colorado. All damage occurred roughly five kilometers weest of the volcano. Lava flows eradicated the town of Pueblo Nuevo, and by the end, 78 people had died. Three new craters were formed during the eruption. Since then Arenal hasn't erupted; it just rumbles and explodes, shooting out rocks, soot and smoke.

Arenal is most impressive at night - in the dark, bursts of fire and red-hot ricks shoot hundreds of feet into the sky. Incandescent material cascades down the sides, especially the north side. In the day you see only smoke and hear the volcano's terrible roar. There are explosions every few hours during Arenal's active phases but it can go mojnths without activity.