How a Schoolboy’s Question Uncovered the 1,200-Year-Old Secret of Gyeongju Bulguksa’s Double Arch

Baegun Bridge and Cheongun Bridge, Bulguksa temple Gyeongju

In front of Daeungjeon Hall, the main hall of Bulguksa Temple, Unesco world heritage, there is a stone staircase called Baegun Bridge and Cheongun Bridge, which lead up to Jahamun Gate.Beneath these stairways lies a hidden architectural marvel — a pair of rainbow-shaped stone arches, known as double arches.

It is said to be the only one of its kind in the world.

Each arch is constructed by stacking inverted trapezoidal stones in a graceful curve, forming a strong arch that beautifully distributes the downward force from above.

However, such arches are naturally weak against upward forces, as they could collapse if pushed from below. To overcome this structural weakness, the ancient builders ingeniously added another arch above the first one, forming a double-arch system.

At the center of the upper arch, they inserted a keystone shaped in the opposite trapezoidal direction, which locks the two arches together so firmly that even if the ground were to heave upward during an earthquake, the structure would remain intact.

This brilliant example of ancient seismic-resistant engineering was built over 1,200 years ago, demonstrating the remarkable skill and ingenuity of Silla-era architects.

The first Korean building to incorporate earthquake-resistant design

This amazing secret was discovered only 51 years ago. At that time, an elementary school student—or a “national school student,” as they were called back then—asked his teacher during a school field trip,

“Teacher, why are the shapes of the stones on the upper and lower arches completely opposite?”

The teacher, intrigued by the question, later contacted experts in the academic field to find an answer. That simple yet insightful question shocked the scientific community, leading to a deeper investigation. It was then—after more than 1,200 years—that the hidden architectural secret of Bulguksa Temple was finally revealed.

  Bulguksa Temple as It Appeared in the Late Joseon Dynasty