Large flashing light extending down from sky to lake Michigan.

Clear evening very little over cast.  Myself and my wife observed a large shatf of light out over lake Michigan.  It looked like a very thick bolt of lightning but continued to pulse about once a second.  It stayed in a genreral area moving back and forth.  It extended down from the sky to the lake and was very bright kind of like a strobe light.  It appeared to be thick in diameter and long given the distance out over the water.  As stated it would flash about once a second and move back and forth in the same general area.  It continued for about 30 minutes when it just stopped.  It did not look like a lighthouse due to the length of it and the way it moved from south to north and back again. I have a brief portion on video that is being sent via mail. I did not film the entire duration unfortunately.