True navel piercings
A true navel piercing is a piercing that passes through the belly button itself. Navel piercings, as the word is commonly used, are through the rim of the navel — skin, which is not actually a part of the navel structure, but a part of your stomach's skin. Naturally, an "outie" (to some extent) is required.
Contrary to urban legend, this will not — except in people with really bad luck (or really bad hygiene) — draw an infection into your liver or peritoneum, and is not generally life-threatening. It is however extremely rare.