Lake County Sheriff's Marine Unit deputies were dispatched to the lake at around 3:50 p.m. for a report of a 52-year-old Mount Prospect man who jumped into the water and did not resurface.
According to police, the man was on a pontoon boat that was being operated by his adult son.
He briefly resurfaced but then fell below the water surface and he was not seen again, police said.
Sheriff's marine unit boats and area fire department boats have been searching the lake with sonar.
The water in the area of the search is about 30 feet deep, according to police.