The diver went missing off the shoreline of Pleasant Prairie in Kenosha County, Wisconsin around 11:30 a.m.
The diver had been exploring an old shipwreck about 6 miles out in water over 100 feet deep.
The Kenosha County Sheriff's Department, the Pleasant Prairie Police Department, the Pleasant Prairie Fire Department, the United States Coast Guard and other agencies have been working together to locate the diver.
The search is expected to resume on Wednesday morning.