Poas Volcano National Park in the north-east of Costa Rica, an hour drive from San Jose. This basaltic volcano stands at an altitude 2708 m and is surrounded by a tropical forest. It has erupted at least 39 times since 1828 and is in a state of nearly continuous mild activity. This lake with the high sulphur content is 1.5 km in diameter and 300 m deep. It's very acidic (pH is less than 1) with water temperature reaching +85 C.
It may take quite a while to capture the lake without steaming gases, but the outcome is worth it - all colors are original.