According to the Ministry of Education and Science, a glass bottle was found 40 centimeters below the surface, which was emitting 40 millisievert/hour radiation. Radium-226 is suspected.
(How many more radium-226 bottles are there buried in Setagaya-ku?)
From Yomiuri Shinbun (11/1/2011):
Regarding the high radiation detected inside the supermarket compound and in the vicinity in Hachimanyama in Setagaya-ku, Tokyo, the Ministry of Education and Science announced on November 1 that the cause of the high radiation was likely to be "radium-226" buried in the soil.
At 40 centimeters below the ground surface, 40 millisieverts (or 40,000 microsievert) per hour radiation was measured, according to the Ministry. 40 millisievert is 40 times the annual radiation exposure limit [still 1 millisievert, though it is likely to change very soon] for the general public [not engaged in radiation work].
According to the Ministry of Education, there was a glass bottle at 40 centimeter below the surface, and the radiation on the spot was 40 millisieverts/hour. The radiation on the ground surface there was 1 millisievert/hour. The work is proceeding very cautiously because the radiation level is extremely high. The bottle will be removed on November 2 or after.
The Ministry and the Setagaya government have been saying "There is no effect on health". Really. (1 millisievert/hour, 24 hours a day, 365 days a year, and we would be talking the radiation exposure in sievert.)