Sinkhole risk in Cook County, IL: Mostly deep or small

Mostly deep or small

Some soluble rock, mostly deeply buried or in a small share of the county.

On soluble rock: 96%
At or near the surface: under 1%
Buried under cover: 95%

The next step for one property

Sinkholes are uncommon here; watch for new depressions after heavy rain or leaking pipes.

That works out to roughly 902 of Cook County's 945 square miles on soluble rock, 5 of them with the rock at or near the surface. With about 5,182,617 residents (5,485 per square mile), there is effectively nowhere in the county off the soluble rock, town or country. The figures come from 551 map points about 1.3 miles apart, so a pocket smaller than that can be missed or overstated.

Worth reading for this county: why the 95% under cover still counts, and how these percentages are measured.

Counties around Cook County

The nearest county rated sinkhole country is Muskegon County, MI, about 123 miles northeast. 5 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Lake County, IL 22 mi north · 0% at surface
DuPage County, IL 23 mi west · 0% at surface
Lake County, IN 32 mi southeast · 0% at surface
Will County, IL 35 mi southwest · 3% at surface
Porter County, IN 40 mi southeast · 0% at surface
Kane County, IL 40 mi west · 0% at surface

The rock underneath

The largest mapped soluble unit here is the Silurian (dolostone (dolomite)), Silurian in age, covering about 96% of the county.

Against the rest of Illinois

Cook County ranks 23 of 102 in Illinois for soluble rock at the surface, and sits above 53% of US counties on that measure.