Irazu Volcano is the highest one in the country!! It´s the only place in America, from which it´s possible to see both oceans at once. Had two catastrophic eruptions, one in 1723 and a second one in 1963, covering San Jose with thick layer of ash.
Arenal Volcano, the most active in Costa Rica. It´s quite young volcano, about 3000 years old. The biggest eruption of the latest years took place on 2001. Since that time on every day there are 3-20 explosions with lava flowing.