Korean Air and Asiana Airlines have accumulated over 4 trillion won in unpaid mileage owed to their customers.

Yet, something seems odd.

Even though the airlines' books show 4 trillion won worth of unused customer mileage, passengers complain that it is nearly impossible to book popular routes or long-distance prestige seats, even by joining the "open run" scramble a year in advance.

With so much mileage piled up, why are there no seats available to use? And how will Asiana's mileage change once the two airlines merge into one?

Will its current value be maintained as is? How will the integrated Korean Air guarantee customers' rights to the 4 trillion won they have accumulated? Also, should people use their mileage now or wait? We looked into it on <Mug Deep>.

(Reported by Sim Yeong-gu | Video by Lee Gi-eun | Design by Ahn Jun-seok | Assistance by Jung Seo-woo | Produced by Knowledge Contents IP Team)