It's an age-old playground question with only two answers: innie or outie?

Well for one Czech underwear model, the answer is "neither".

Karolina Kurkova, 24, is making waves after appearing on a catwalk in Miami Beach without a belly button. The missing navel has now become, in the words of one paper, "the riddle of the £2.5 million beauty".

Some people naturally lack their belly-button as a result of abdominal problems shortly after birth, a condition known as gastroschisis. However some have speculated that Kurkova's rounded navel may in fact be the result of a cosmetic proceedure. Doctors have turned innies into outies (and vice versa) for years -- surely covering it up couldn't be that hard?

For now the mystery remains just that -- a mystery. Kurkova has not talked about it publicly, and all her agent will say -- according to the BBC -- is that she is "not an alien." (Note that the agent did not rule out that Kurkova may be a sea monster, angel or computer-generated hologram... We're just saying.)

What do you think? Is the belly-buttonless naval attractive or not? Take a look at our gallery below, and then let us know what you think in the comments.