The Soviets had a placed a thermonuclear device just beneath the surface of a river bed. A 5 kt fission primary provided the energy to initiate the fusion reaction.....
Booom!
A crater 408 m in diameter and 100 m deep remained. The rim of the crater, over 20 m high, prevented any water from getting in until bulldozers cleared a path. Ten million cubic meters of water was trapped to form a lake.
Radioactive dust fell within a radius of 8 kilometers.
Little was done to protect the local people who were allowed back to their homes after a few days. The surrounding land is still contaminated.
Initially the radiation dosage at the edge of the lake was about 20 R/h (Roentgens per hour) mainly due Co-60. This isotope has a half-life of about five years and today the radiation level has dropped to about 2 mR/hr but the legacy of this blast remain.
In an effort to assure the public of its safety, various Russian officials have swum in Lake Chagan (or 'Atomkul'). The newly independent Kazakhs will for a long time be burdened by the nuclear testing that took place at Semipalatinsk.
source: www.fas.org/nuke/hew/Russia/sovwpnprg.html | 
