WHILE EN ROUTE FROM ONE PRIVATE AIRSTRIP TO ANOTHER A SHORT DISTANCE AWAY, THE AIRPLANE COLLIDED WITH ROLLING TERRAIN IN A NOSE DOWN ATTITUDE. A WITNESS REPORTED THAT HE HEARD AN AIRPLANE CIRCLING THE AREA, THEN HE HEARD A THUD. WHEN HE WENT TO INVESTIGATE, THE AREA WAS COVERED WITH DENSE FOG AND VISIBILITY WAS LIMITED TO 75 FEET. THERE WAS NO EVIDENCE TO INDICATE A MECHANICAL FAILURE OR MALFUNCTION.