How SinkholeOdds rates a county
One national map, measured county by county
The source is the USGS Karst Map of the United States (Weary and Doctor, 2014, Open-File Report 2014-1156). It draws every mapped body of soluble rock in the lower 48 states and records what kind it is, what climate it sits in, and how thickly it is covered. For each of 3,109 counties, SinkholeOdds lays a regular grid of points (about 380 per county on average) inside the 2023 Census county boundary and checks which karst layer each point falls on. The share of points is the share of land.
The four numbers on each page
On soluble rock is the share of the county over carbonate rock (limestone, dolomite, marble) or evaporite (gypsum, salt) of any depth. At or near the surface is the part the USGS marks as exposed or nearly so. Buried under cover is the part under sediment or glacial till. The page also names the largest mapped unit, so you can look it up in your state survey's records.
How the tier is set
No agency grades counties for sinkholes, so the tiers are this site's own split of the data. Sinkhole country means at least half the county has soluble rock at or near the surface in a humid climate, or at least a quarter sits on gypsum or salt. Some sinkhole ground means at least 15% at the surface, or more than half under thin cover. Below that, a county with any soluble rock is low. Today that gives 386 counties in the top tier and 648 in the second, home to about 94.8 million people combined. The states with the most top-tier counties are Missouri (56), Tennessee (48), Kentucky (40), Florida (26), Texas (24). 1,098 counties have no soluble rock mapped.
What the data cannot tell you
The map shows rock, not holes. It does not count sinkholes that have formed, and it cannot see a void under one lot. A grid point lands every mile or so in a typical county, so a narrow limestone band can be missed or overstated. Alaska and Hawaii are not included yet. Use the tier to decide whether to look harder, then use your state survey's sinkhole inventory and an inspection for the lot itself; see signs of a sinkhole and how inspection works.
Where to go next
See how it varies: Izard County, AR, Southampton County, VA, Elmore County, AL.
Why "buried" limestone is not automatically safe · Gypsum and salt: the rock that dissolves fastest