A more thorough explanation:

An abnormally dangerous activity is a legal term used in tort law. It refers to an activity that:

1. Is not commonly used
2. Creates a foreseeable and significant risk of physical harm, even when reasonable care is taken by all parties involved

If a person is found to have engaged in an abnormally dangerous activity, they may be held strictly liable for any physical harm that results from that activity.

The rule on abnormally dangerous activities originated from the English case Rylands v. Fletcher in 1868. American courts have since adopted the rule, but with some modifications. For example, while English courts required the activity to take place on the defendant's land, American courts do not have this requirement.

There are two key elements to determining whether an activity is abnormally dangerous:

1. Common usage: An activity is not abnormally dangerous if it is commonly used in the community. For example, driving a car is a common activity, but storing toxic chemicals is not.
2. Highly significant risk of physical harm: An activity is abnormally dangerous if it poses a significant risk of physical harm, even if reasonable care is taken. The severity of the potential harm and the likelihood of that harm occurring are both considered.

For example, using explosives in a residential neighborhood poses a highly significant risk of physical harm, even if reasonable care is taken. On the other hand, driving a car, while not risk-free, is not considered abnormally dangerous because it is a common activity and the risk of physical harm is not unusually high.

Overall, an abnormally dangerous activity is one that poses a significant risk of physical harm, even if reasonable care is taken, and is not commonly used in the community.