Chinese artillery officers are puzzling over a recent incident. On October 1st, an artillery shell, fired from North Korea, landed next to a government building in northern China. The shell did not explode, but buried itself about three feet in the earth, and created a crater about six feet wide. Soldiers dug the shell out, and artillery experts disassembled it. The shell did not have a fuze, and instead of explosives, the interior was filled with an inert substance that was part cement. Speculation is that the poverty stricken North Koreans leave explosives out of practice shells, to save money, and enable them to do practice firing close to occupied areas. There are several North Korean army bases, close to the Chinese border, that have artillery units in them. Less charitable speculation was that the shell was a test run for a new smuggling scheme, firing shells filled with heroin (which North Korea has been producing for years, as a way to raise badly needed foreign currency.) The police have been questioning people along the border, to see if there may have been other shells, of this type, that landed. Police have also warned people not to talk about the incident, but that obviously didn't work.
These shells make a loud "thump" when they land, and a noise similar to a train passing overhead, if you are under the flight path. But if the shells are fired into thinly inhabited areas, no one would notice. Unless they knew where to look. An experienced crew can fire 122mm or 152mm shells quite accurately, for long distances (20 kilometers or more.)