Kilju, North Hamgyong Province, Democratic People's Republic of Korea (DPRK; North Korea).
~0950hrs 25.May 2009
Seismic indications of a substantial underground explosion.
Looks like they managed a multi-kT device this time (multi-kT = in excess of 1 kilo-ton of TNT in blasting power; for comparison, the WWII-era weapons were between 15 and 21 kT).
Yonhap News (South Korea) also has sources saying the P.R.Chinese were informed before the test.