Excerpts from abc7chicago.com:

Libertyville police and firefighters responded around 4:45 p.m. Monday (6/4/18) to several 911 calls about a car floating in a large pond near US Highway 45 and Northwind Boulevard. The callers said the driver was still inside. Firefighters tried to put a lifejacket on the driver and pull him out, but a diver was sent in to rescue him when the car started to sink.
Fire officials said that at one point the car’s trunk opened, which caused the car to sink quickly, and straight down.

Though divers managed to get him out, he was rushed to Advocate Condell Medical Center, where he was pronounced dead.  The cause of the crash remains under investigation.