system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we keep our belly buttons for life?

Why do belly buttons heal and leave behind a hole (or bump) that never goes away? Shouldn't they leave a surface scar on the skin that eventually fades like other injuries?
gpt
Nothing on your body will regrow after you've been born - at that point the basic layout of your body is set in stone and nothing that's missing or damaged will regenerate beyond creating scar tissue.

The umbilical cord tunnels all the way through your abdomen, extending through your skin and subcutaneous fat.  Because of its position, that skin and fat just never forms.  The skin will "heal" through the formation of a scar, so you don't end up with a big hole in your stomach (though that's not really the same thing as healthy skin).  But there's no similar process to fill the space of the underlying fat, which causes your belly button to dip inward into the gap caused by the missing fat.

Other injuries that destroy subcutaneous fat later in life, such as animal bites, produce scars that look like shallow belly buttons.  These injuries destroy the subcutaneous fat, which never comes back.  The scar tissue that forms above then "falls" in to the hole left by that missing fat, creating a pit like scar.

The belly button happens to be really prominent because of how much fat there is surrounding it, which results in an extremely deep pit.  But again, any scar that destroys the underlying fat will look like that down to the depth of the surrounding fat in the area of the scar.