On Tuesday North Korea tested an intercontinental ballistic missile by firing it over Japan.
The move was described by Japan's prime minister Shinzō Abe as an 'unprecedented threat'.
This was the first time a ballistic weapon has been launched over the island nation.
According to the BBC, previous incursions into Japanese airspace by missiles in 1998 and 2000, were allegedly non-weapons designed to launch satellites from North Korea.
Reports from the South Korean military claim Tuesday's missile broke into three pieces, and crashed into the sea, roughly 1,180 km from the Japanese shoreline.
The projectile was the newly developed 'Hwasong-12' intermediate range missile.
On Wednesday, North Korea's leader Kim Jong-un said the missile test was the first step to 'containing Guam'.
Japan uses an advanced emergency alert system to warn citizens about potentially lethal events, such as earthquakes, nuclear meltdowns, and - in this case - missiles overhead.
One thread on reddit asks people living in Japan how they felt when they received the alert.