What is the Kotlin double-bang (!!) operator?

I'm converting Java to Kotlin with Android Studio. I get double bang after the instance variable. What is the double bang and more importantly where is this documented?

mMap!!.addMarker(MarkerOptions().position(london).title("Marker in London"))

This is unsafe nullable type (T?) conversion to a non-nullable type (T), !! will throw NullPointerException if the value is null.

It is documented here along with Kotlin means of null-safety.

Here is an example to make things clearer. Say you have this function

fun main(args: Array<String>) {
    var email: String
    email = null
    println(email)
}

This will produce the following compilation error.

Null can not be a value of a non-null type String

Now you can prevent that by adding a question mark to the String type to make it nullable.

So we have

fun main(args: Array<String>) {
    var email: String?
    email = null
    println(email)
}

This produces a result of

null

Now if we want the function to throw an exception when the value of email is null, we can add two exclamations at the end of email. Like this

fun main(args: Array<String>) {
    var email: String?
    email = null
    println(email!!)
}

This will throw a KotlinNullPointerException