They’re referring to landmines, leftover from that region’s wars of the mid-20th century: America’s Vietnam & Cambodian war, French-Indochina war, Cambodian civil war, etc.
The legacy of landmines and chemical warfare is still regularly killing, maiming, and causes significant increases in fetal birth defects and other rare illnesses.
Japanese cities were primarily built using wood as it was better suited for their climate and earthquakes. The fire bombing of Tokyo with a single deadliest attack on the Japanese mainland, killing even more than either atomic bomb drop.