At the rim there are two craters.  The main crater measures 1.5 kms. in diameter and is 300 meters deep.  It has a circular hot-water lake at the bottom, 350 meters in diameter, and a cinder cone 40 meters high with very active fumaroles.  The second crater is the seat of a cold–water lake, Botos, which clearly originates from rainfall and drains into the Caribbean by means of the River Angel, a tributary of the River Sarapiquí.

Poas Volcano has a long history of eruptions that range from emissions of hot vapors and muddy water to blowouts of ash and incandescent rocks.  On 25^th January 1910 an enormous column of ash that was over 8,000 meters high was visible from several different places around the country.  The last period of violent eruptions with large clouds of ash and incandescent rocks took place between 1952-1954. Because of its geyser like eruptions when columns of muddy water and steam shoot several meters up into the air, Poas is known as the largest geyser in the world.  This is a phenomenon that can be repeated within a period of several minutes or not take place again for several years.