The piercing of the belly button is completely Haram and forbidden in Islam and will constitute a grave sin

The piercing of the belly button will involve the showing of the Satar which is completely Haram and forbidden in Islam and will constitute a grave sin, on the one showing and on the one looking.

Evidence is found in the Hadith (traditions of Rasullulah) that the abhorrent sinful women would have such changes, disfigurations and alterations made to their bodies and change Allah's creation made in order to look beautiful and pleasing to men who used to come to them.

The Hadith narrated by 'Abdullah ibn Mas'ood says that: "I heard Rasulullah say: "Allah has cursed the woman who does tattoos and the one who has them done, the woman who plucks eyebrows and the one who has it done, and the one who files her teeth for the purpose of beauty, alteringthe creation of Allah."
(Sahee Al-Bukhari, Sahee Muslim Vol 2)

The warning and curse of Allah in this Hadith is also applicable to belly button and other bodily piercing, other than the ears and nose. Allah the Almight says in the Quran: "And do not cast yourselves into destruction by your own hands."
(Al-Baqarah: Verse 195)

"And I (Satan) will surely lead them astray, and arouse desires in them, and command them so that they will cut the cattle's ears, and I will surely command them and they will change Allah's creation.' Whoever chooses the Devil for a friend instead of Allah is assuredly a loser, and his loss is manifest."
(An-Nisa': Verse 119)

To have other body parts pierced other than the ears and nose is mutilation (muthlah) of the human body which is forbidden in Islam. Both the person having it done and the doer will be in sin. Mutilation of this sort will indeed cause a great deal of harm to the body. It is highly possible that it could be a means of infection to cause irreversible harm to the body. This is not promoted in Islam but forbidden.

It is also an imitation of the Non-Muslims (who invented this act and made it into a fashion) and has been highly popularized by actors and actresses as an object of beauty. In light of this it will be "tashabuh" (imitation) of a non-Muslim nation.

Rasullulah Salaam Alaihi Wassalam has said," "Whosoever impersonates a nation (other than Islam) will be (resurrected) from them on the day of judgment". (Sunan Abu Dawud)

Therefore, in conclusion one can say that it is strongly advised to avoid piercing the belly button and various other parts of the body, with the ears and nose been exempted from this. It is also Haram to open ones Satar to anyone (other than that husband or a female physician or for other medical reasons) and doing so will incur great sin upon the person. In addition it is forbidden for males to pierce any part of their bodies.

And Allah Taala knows best