A dead drop or dead letterbox is a location which two people can use to pass off information without ever meeting in person. A classic dead drop is a commonplace public location where comings and goings would not be observed, with information being hidden in things like trash cans, toilet tanks, holes in tree trunks, and so forth. The espionage community has also developed more sophisticated dead drop systems which rely heavily on technology, theoretically making them more challenging to identify.
There are a number of advantages to using a dead drop, which explains why dead drops have been used for centuries by people who want to pass information quietly and efficiently between each other. One major advantage to using a public dead drop instead of meeting to exchange information is that the users of the dead drop can minimize the appearance of suspicious activity. For example, taking a walk in a park and sitting on a bench now and then is a perfectly normal activity which might not arouse much suspicion, whereas ducking into a bush and talking with someone could be perceived as unusual.
Dead drops can also be used to protect the members of an espionage group. For example, two cells could pass information through a dead drop without ever meeting each other, thus ensuring that if members of either cell are captured, they cannot disclose the identity of the members of the other cell. Users of the dead drop can also visit at any time, thus avoiding a regular schedule which could be noted by observers.
When people leave information in a dead drop, they often leave signals in the area to alert the person they are communicating with to the fact that there is something in the dead drop. These signals can take the form of chalk marks, strategically placed rocks, and other subtle changes in the environment which would be difficult for outside observers to interpret. Clever agents also change their signals and dead drop locations periodically, to avoid the creation of a pattern which could be used to identify them.
The modern dead drop tends to be much more sophisticated than a knothole in a tree. Many dead drops today are entirely electronic, creating a situation where no one needs to visit a physical site. For example, agents might use a wireless router as a dead drop, encoding messages which can be picked up by users who know how to access them.
