The explosion at an annex of a women-only spa located 500 meters from Tokyo's Shibuya Station, which killed three women and injured eight people, has taught many Tokyoites something: Drill deep enough underground in Tokyo and you'll likely strike hot water containing methane gas. That's because the city sits on the Southern Kanto Gas Field. This geological feature has fostered 148 spa facilities in the capital.

At the spa facility, the water was piped from 1,500 meters underground. Its annex building was equipped with a device that separates methane gas from hot water and releases it into the atmosphere. It is suspected that methane gas trapped inside the annex building exploded. The building housed changing rooms for spa employees on the first floor and a pump to raise hot water in the basement. Five of the 11 casualties were spa employees. If the pump and the gas separator had been installed inside the main building, the number of casualties could have been larger.

There is a strong possibility that executives of the spa operator were unaware of the danger of methane gas. Although the firm says that the annex basement was equipped with a gas detector, police have yet to find it. Police say that since the spa facility opened in January last year, the firm had never examined the gas separator nor measured the methane gas concentration. Police must carry out a strict investigation to determine whether there was negligence on the part of the operator and whether the design of the annex was appropriate.