Reading the following Scriptures will demonstrate that this unknown tongue is not the same as the gift of tongues.

The Unknown Tongue

1 Corinthians 14:2 For he that speaketh in an unknown tongue speaketh not to men, but to God: for no man understandeth him; howbeit in the spirit he speaketh mysteries.

14:3 But he that prophesieth speaketh to men to edification, and exhortation, and comfort.

14:4 He that speaketh in an unknown tongue edifieth himself; but he that prophesieth edifieth the church.

14:5 I would that ye all spake with tongues, but rather that ye prophesied: for greater is he that prophesieth than he that speaketh with tongues, except he interpret, that the church may receive edifying.

14:6 Now, brethren, if I come to you speaking with tongues, what shall I profit you, except I shall speak to you either by revelation, or by knowledge, or by prophesying, or by doctrine?

1 Corinthians 14:14-15 For if I pray in an [unknown] tongue, my spirit prayeth, but my understanding is unfruitful.  What is it then? I will pray with the spirit, and I will pray with the understanding also: I will sing with the spirit, and I will sing with the understanding also.

1 Corinthians 14:18-19 I thank my God, I speak with tongues more than ye all:  Yet in the church I had rather speak five words with my understanding, that [by my voice] I might teach others also, than ten thousand words in an [unknown] tongue. 

The Gift of Tongues:

In the midst of all the statements above, Paul says this:

1 Corinthians 14:5 I would that ye all spake with tongues, but rather that ye prophesied: for greater is he that prophesieth than he that speaketh with tongues, except he interpret, that the church may receive edifying.

1 Corinthians 14:13 Wherefore let him that speaketh in an unknown tongue pray that he may interpret.

1 Corinthians 14:22 Wherefore tongues are for a sign, not to them that believe, but to them that believe not: but prophesying serveth not for them that believe not, but for them which believe.

14:23 If therefore the whole church be come together into one place, and all speak with tongues, and there come in those that are unlearned, or unbelievers, will they not say that ye are mad?

This would all seem to be contradictory and allow for much speculation, but God has given some experience in these last days.  When a man goes into a group of people but does not speak their language, though they can speak his language, and when this man who does not know their language speaks to them in their own language then perfectly gives the interpretation in his own language, it is a sign to these unbelieving people.  That can plainly see that the message must be true since the man could not have given it in their language unless God had performed the miracle of the gift of tongues and interpretation.  Though we would like to have more authority in these gifts, we have seen them operate in the church.  It is a sign to the unbelievers when this happens.  If there were no interpretation and no one knew what the person was saying, then it would be a disorder in the church and the unbelievers would think that the speaker was a crazy person.

See also:

1 Corinthians 12:10 To another the working of miracles; to another prophecy; to another discerning of spirits; to another divers kinds of tongues; to another the interpretation of tongues:

1 Corinthians 13:1 Though I speak with the tongues of men and of angels, and have not charity, I am become as sounding brass, or a tinkling cymbal.

Note the two types of tongues.  They are tongues of men and the tongues of angels.  Both are good.  They also can be misused as any good gift can be misused.  They also can be counterfeited, just as any good gift can be counterfeited.