The vulnerability of Tokyo's bay front has been revealed when on July 2, the Japanese-operated super-tanker Diamond Grace , struck an undersea reef in the bay and spilt about 1,300 tonnes of crude oil. The crude-oil spill, the fifth worst in Japan's history, left the residents and industries around the area worried about the possibility of future spills and put pressure on shipping lines to speed up the production of double-hall tankers. The crude-oil spill also put the bay's congested waterways in the public spotlight. The captain of the Diamond Grace said that the ship was trying to steer clear of two fishing boats and three large vessels at the time of the accident.

About 60 per cent of the bay is less than 4.572 m deep and about 100 tankers pass through each day. The number of ships passing through per day was 600 in 1994 and 700 in 1995.Some analysts argue that large tankers like Diamond Grace often pose the problem of scraping the bottom when they travel through the shallow bay.

The Tokyo Metropolitan Fire Department said that within 24 hours of the spill, it received 453 complaints about a nauseating odour emanating from the bay. In Tokyo, 18 people were taken to hospital as they fell ill as a result of the smell, said a metropolitan government official.

Fisherfolk were forced to suspend some of their operations. Usually, fisherfolk catch flatfish, goby and other fish in the bay. Some 340 ships were mobilised for the clean-up operation. Yokohama, located just south of Tokyo, is Japan's second-largest city and a major port. Meanwhile, the municipal officials who conducted tests around Yokohama' port areas said that the oil spill had little effect on the waters.