{
gSystem->Load("libGeom"); // load the geometry library
TGeoManager *mng = new TGeoManager("world", "the simplest geometry");

TGeoMaterial *mat = new TGeoMaterial("Vacuum",0,0,0);
TGeoMedium *med = new TGeoMedium("Vacuum",1,mat);
TGeoVolume *top = gGeoManager->MakeBox("Top",med,10.,10.,10.);
gGeoManager->SetTopVolume(top);   // accessing the world

gGeoManager->CloseGeometry();     // via a global symbol

top->SetLineColor(kMagenta);
mng->SetTopVisible(); // the TOP is generally invisible
top->Draw();
}