Ball lost in boggy area

What is the nature of the area surrounding that GUR? closely-mown? rough (how deep)? etc. Could the ball be anywhere else that it might not be found?
In order to proceed under the GUR Rule in the situation that you describe (ball not found), it must be known or virtually certain that the ball is in the GUR. "Known" is pretty obvious - saw it go in there or find the ball in there. "Virtually certain" means 95% probability that the ball is in there (same as 19 times out of 20).