Extremely long, obstructed, or very violent rapids which expose a paddler to added risk. Rapids may continue for long distances between pools, demanding a high level of fitness. What eddies exist may be small, turbulent, or difficult to reach. At the high end of the scale, several of these factors may be combined. Scouting recommended but may be difficult.
Swims are dangerous, and rescue is often difficult even for experts.
Rapids at the upper end are classed as 5.1, 5.2, etc. These runs have almost never been attempted and often exemplify the extremes of difficulty, unpredictability and danger. The consequences of errors are very severe and rescue may be impossible. For teams of experts only, at favorable water levels, after close personal inspection and taking all precautions. After a Class VI rapids has been run many times, it's rating may be changed to an appropriate Class 5.x rating.