Speaking in an unknown language. That is to say it is unkown to the speaker but is not unknown to God. Speaking in tongues began on the day of Pentacost by Jesus' disciples. The purpose of speaking in tongues (in my view) is to issue prayer that encompasses issues of life unknown, but needing to be dealt with. Speaking in tongues may seem foolish to the average person (even the average Christian), but if you desire wisdom, it is the thing to do.
The Holy Spirit's indwelling is made aware by the evidence of speaking in tongues