At one point, he dangles by one hand from a steel beam at the center of the chimney as pieces of wood become dislodged, falling for 10 seconds before the thud into the chimney's dark abyss.
Even more terrifying might be the climb down. Cernescu and his companion make their way from the top on a ladder that looks like it could become dislodged at any moment. In fact, it looks like it already did near the top. The ladder is held "in place" by a rope tied to a higher level. Additionally, they make the climb down with no harness. The harness you see is just tethering the unicycle to his waist.
It's not the only time he's done stunts at this location. His YouTube channel — unsuitable for anyone with a fear of heights — has him slacklining across the center of the chimney in early August. It's not as flashy, but equally as terrifying.