You may have noticed an odd dark line running down from your belly button - but how did it get there in the first place?
There are a lot of things that we don't know about the human body, but with this one bizarre feature being quite visible, you might wonder why it's a thing at all.
First of all, not everyone has the line, so if you've just had a look and seen nothing there, you might be a bit confused with what we're on about.
You may also think that it's only seen on woman that are or have been pregnant, but again, this is not the case.
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It's a lot more nuanced than that, with health factors meaning that men also able to sport this faint line from their belly button.
What is it called?
It's called the linea nigra, or 'black line', and according to Cleveland Clinic, it can be defined as a dark vertical line that appears on the skin of your stomach during pregnancy - it's even nicknamed 'the pregnancy line'.
It runs down to your pubic region, and can even extend up to your abdomen.
The line can appear darker as the pregnancy progresses, and is more visible in those with darker complexions.
Why does it appear at all?
Let's start with why it appears during pregnancy - the short answer is hormones.
Basically, everyone has a linea nigra, but it's almost invisible until your hormones increase during pregnancy, often appearing in the second trimester.
When it isn't seen, it is known as linea alba, or 'white line'.
As for its appearance when you're not pregnant, doctors have theorised that its to do with hormones once again, according to Healthline.
The effects of hormones on linea nigra
It's well known that hormones can contribute to a number of changes in our bodies, pregnant or not.
Healthline state that the linea nigra is a result of estrogen and progesterone's effects.
These hormones cause the body's melanin-producing cells to form more melanin, and melanin is responsible for darker skin tones and tans.
As your melanin increases, your skin darkens, which can include the lighter parts of your skin - such as the linea alba.
What else could cause linea nigra?
Environment and medication could play a role, with the likes of birth control or health conditions altering hormone levels.
An increased exposure to the sun could also increase melanin production, making some sections of your skin darker than others.
Does everyone get a linea nigra?
When it comes to pregnant women, it is estimated that around 80 percent will have a linea nigra, though it is dependant on your complexion as well.
The line usually fades after pregnancy when hormone levels return to normal, though it can stay for certain people.
No-one really knows what the purpose behind the line is though, so at the end of the day it's just a line that some have, and some don't.
Joshua Nair