/ˈʌltrəˈhæzərdəs ˌækˈtɪvəti/ – Phrase
Definition: Hoạt động cực kỳ nguy hiểm.
A more throughout explanation: an activity, action or process that is so dangerous that the person or entity conducting the activity is strictly liable (=legally responsible for something without having done anything for which you are personally to blame) for any injury caused as a result of that activity
Example: Some people consider bungee jumping an ultrahazardous activity.