An ultrahazardous activity in the common law of torts is one that is so inherently dangerous that a person engaged in such an activity can be held strictly liable for injuries caused to another person, even if the person engaged in the activity took every reasonable precaution to prevent others from being injured.

There are several categories of activities which are commonly recognized as being ultrahazardous, and therefore subject those who engage in them to strict liability. These include:

• Transportation, storage, and use of dynamite and other explosives
• Transportation, storage, and use of radioactive materials
• Transportation, storage, and use of certain hazardous chemicals
• Keeping of wild animals (i.e. animals that are not normally domesticated in that area)
  • Note that in this context domesticated does not merely refer to animals that are commonly bred and raised in captivity, such as alligators.
• Keeping of domesticated animals that have a known propensity for dangerous behavior (e.g., keeping a dog that has attacked people before)

A person who is injured by one of these ultrahazardous activities while trespassing on the property of the person engaged in the activity is barred from suing under a strict liability theory. Instead, they must prove that the property owner was negligent.

In the United Kingdom, this area of law is governed by the rule established in Rylands v Fletcher.

Factors determining an activity is ultrahazardous:

1. Large possibility of harm
2. Level of seriousness of potential harm
3. Activity is not one of common usage-- the average person regularly would not engage in it
4. Possibility of harm is not decreased with the utmost care
5. Social value of the activity does not outweigh the risk
6. Inappropriateness of the activity in the area it is commenced

Tort law