Edogawa Ward didn't mince words when it updated its hazard map in May for the first time in 11 years.

"Do not remain here," it said in bold letters, urging residents to flee from the ward to other areas in the event of wide-scale flooding. The municipality has reason to be concerned: sandwiched between the Arakawa and Edogawa rivers and facing Tokyo Bay, 70 percent of its land is below sea level, making it one of the most vulnerable among the capital's 23 wards to disasters caused by torrential rain, typhoons and high tides.

It's not only Edogawa. Most areas in the low-lying "five wards of Koto" in eastern Tokyo — also including Adachi, Katsushika, Koto and Sumida — will be submerged in a worst-case scenario, it said, affecting 2.5 million people, or over 90 percent of the entire population of the five wards combined.