An auger generally comprising a shaft having flighting which is helically wound around the shaft to convey spoil to the surface of the area being excavated. The shaft has a first end and a second end. The shaft first end is adapted to be combined with and rotated by a drilling machine. The shaft second end is adapted to be removably combined with a boring head and one or more drilling elements to bore through the ground. The invention comprises two interchangeable boring heads. A first boring head is adapted to drill through soft ground conditions such as dirt or clay and a second boring head adapted to drill through hard ground conditions such as rock.