der Grund — the ground, and also the reason. Aus welchem Grund? means for what reason, literally from what ground.
einer Sache auf den Grund gehen — to go to the ground of a matter, to investigate it to the bottom.
die Ursache — the cause. Ur means original, and Sache means the matter.
die Ursachenforschung — cause research, the investigation of why something happened. Die Ursachenforschung läuft, the search for the cause is under way, is a common line in news reports.
das Übel an der Wurzel packen — to grab the evil by the root.