Tokyo's rainy days fall most often during the Tsuyu season. Tsuyu describes the plum rain front, an annual meteorological event observed across Japan, Korea, China, and Taiwan. It gets its name from the timing of the rains, which coincides with the season of plums ripening. While Tsuyu's timing differs across Japan, Tokyo traditionally sees these rains from early June to mid-late July. However, torrential downpours and extended rainy periods now occur more unpredictably throughout the year.

Warm, moist air from the Pacific Ocean meets the cooler, dry air from the Asian continent, bringing a prolonged season of dampness and humidity. Rain clouds move steadily from the south to the north of the Japanese archipelago during a time of year that leaves a nostalgic imprint despite the discomfort brought on by the weather. Hydrangeas and irises bloom dramatically, and television weather forecasters reach for the term ōame, a compound of the kanji characters for 'big' and 'rain'. On big rain days—and more so when a tropical storm or typhoon looms—complex and vast systems stand at the ready beneath the city's streets. It's a detail of which most residents remain blissfully unaware, yet it is designed to protect them.

Should sensors around roadside ventilation shafts near Tokyo Metro station entrances detect a critical risk of flooding, watertight shields will slide into action, preventing water from inundating the station. Doors capable of completely sealing the opening are positioned behind these shields. If these measures fail to some extent, flood gates in the network's crucial tunnel sections will activate, diverting the water flow to massive underground chambers with integrated pumps ready to expel water swiftly. Detailed infographics found throughout Tokyo Metro's stations and carriages, predominantly in Japanese but rich in illustrative detail, offer insight into this sophisticated system. They read like an intense, multidimensional version of the water cycle diagram.

Since its inauguration in 1927, the Tokyo Metro has drawn insights from the pioneering subway systems of London, Paris, and New York. Japan's knack for integrating and refining foreign technologies is well-documented across numerous sectors, and the Tokyo Metro is no exception. However, it is not merely an imitation; it is a network finely tuned to Tokyo's specific requirements, such as its dense population and need for earthquake-resistant infrastructure, particularly following the devastating Great Kanto earthquake just four years prior. Preparing for the worst has been integral to the Tokyo Metro's blueprint since its inception. Even in the 1950s, as Japan was recovering from the war, a significant portion of the national budget was devoted to disaster prevention and risk reduction.

Closer to my home, by the tranquil banks of the Thames in South West London, riverside homes feature mini front doors starting around a metre above the ground, with steps leading up to thwart water ingress. There is something comical about these doors and the challenges they pose for entry and exit. On days when the Thames turns tempestuous, residents might draft in sandbags at their frontages for an added layer of defence. While Tokyo's proactive stance on disaster preparedness may not be as deeply rooted in other global metropolises, challenges still loom: Paris faces the Seine's potential for dramatic overflows, engineers in London race to mitigate the London Underground's vulnerability to flooding, and New York braces for the next inevitable hurricane.