The search continues for a missing man who witnesses say fell from a sailboat into Lake Champlain on Monday.

Police say Barry Nelson, 73, was sailing around the mouth of the Winooski River when witnesses saw him fall from his boat and never resurface.

Vermont State Police joined the search Tuesday. Divers could be seen making several trips out on the lake. VSP’s rescue boats use side-scan sonar technology to help show what’s in the water.

Police aren’t exactly sure why Nelson fell. They say one bystander saw his boat drifting and say Nelson appeared to hop out to push it, while another says Nelson was clearing water out of the boat when he fell.

Police say the biggest challenge is visibility. BPD’S deputy chief, Wade Lebreque, says the water is only about 10 to 12 feet deep. He was down there Monday and describes it as swimming around in a chocolate milkshake.

“Especially the closer you get to the bridge, the number of trees that are on the bottom, and the limbs, and fishing line and stuff like that creates a lot of hazards for the divers,” he said. “Especially when you can’t see.”

Police say the search will continue Wednesday.