We spent the entire morning and part of the afternoon exploring Sierra Negra, specifically the sulfur mine area in the southwestern corner inside the caldera. The caldera is considered one of the largest in the world. It was an adventurous ride up the flank of the volcano in a local chiva (open-sided bus used all over Ecuador in rural areas). Then we walked, one way two kilometers, which included a descent of 320 feet, then an ascent to the sulfur fumaroles of another 200 feet. On return, we did it all again!