Laguna Caliente (Hot Lagoon) is one of the most acidic natural lakes in the world. The pH of this lake often is close to 0 and no aquatic life has been found in it. The bottom of the lake contains a layer of liquid sulphur. The depth of the lake is very variable - sometimes 40 - 50 m deep, but sometimes it disappears entirely. The diameter of the lake may reach 250 - 300 m. The crater is filled with brine, which is usually 22°C but sometimes it rises up to 94°C. What's fascinating is the diversity of the lake's colors. The lake might be emerald green but some three hours later - grey-whitish. Sometimes rafts of brilliant, yellow sulphur can be seen floating in the lake. As if this is not spectacular enough - Laguna Caliente often experiences impressive phreatic and phreatomagmatic eruptions. This is caused by the magma reaching the water - extreme heat instantly turns the water and acid into explosive steam and the lake blows up like a giant geyser.

Most of the people who visit Poás Volcano do not get to see the main crater because of the thick cloud cover that rolls in almost every day. The best time to see the volcano is at 8am.