Scientists photograph mysterious craters found at the bottom of Lake Michigan

CHICAGO (CBS) -- A mystery lurks at the bottom of Lake Michigan—and it's not a shipwreck, although there are many of those down there too.

About 14 miles off the coast of Sheboygan, Wisconsin, and about 450 feet below the surface, scientists two years ago discovered dozens of massive sinkholes.

This past August, scientists finally got up-close pictures of the sinkholes—and found they range in size from 300 to 600 feet across.

Scientists say dark, low-oxygen, and sulfur-rich conditions in the sinkholes are similar to early conditions on earth—and provide a valuable opportunity for research.

Speaking to Caitlin Looby for a report in the Milwaukee Journal Sentinel , scientists said the craters formed as groundwater formed caverns under the limestone bedrock at the bottom of the lake—and parts of the ceilings of those caverns went on to collapse.

Scientists told the newspaper that few creatures can live in the dark, cold sinkholes—which are constantly about 38 degrees Fahrenheit, or six degrees above freezing—but freshwater opossum shrimp, and a small fish called the deepwater sculpin, and invasive quagga mussels can be found, along with as-yet-unknown bacteria.