FRANKLIN - The sinkhole on High St. remains a problem to the borough, partly because no one is sure just how deep it is, according to Mayor Doug Kistle. "Do we have any bedrock?" Kistle wondered Tuesday. "I'm told there's loose material from long ago, but somewhere in there, there's got to be bedrock. Got to be. It can't be loose 100 feet down." The sinkhole, which is some three feet in diameter and about five feet deep, first became noticeable during the winter months. It has caused much of High St. to be closed to traffic since last week, and the mayor has said it will remain closed until further notice, pending a determination by Elizabethtown Gas as to the status of an exposed gas line in the hole. Because High St. is in an area of town that was heavily mined decades ago by the since-defunct New Jersey Zinc Co., officials believe the hole is atop an old mine shaft. The depression has occurred before, and it is believed that heavy spring rains helped open it again. "We need to get Elizabethtown Gas up here to give us their suggestions as to what they want to do," said Kistle, who does not believe the exposed gas line represents a serious problem. "Of course, a lot of the responsibility is ours, too." Kistle wants the town to determine the exact depth of the sinkhole. He suggested using fire hoses to "blow water in there to see what we have." The mayor is hoping that if the hole narrows as it extends deeper into the ground, "we could build a concrete pad down there to stop it, and then plug this up again." High St. is believed to have once been one of the town's highest streets until all of the mining underneath caused part of it to drop downward. The hole first appeared during the winter and was twice filled with rock and gravel, the mayor said. At that time, the hole was smaller, but heavy rains have since enlarged it to its current size.