A sinkhole is ground collapsing into a void, usually in limestone or other soluble rock. The word gets used for any dip in a yard. Most dips are poor compaction, a buried stump, or a leaking pipe washing soil out. Some dips, in karst country, are the surface of a real void. The difference is geology plus an investigation, not a feeling about a crack in the garage slab.

Foundation cracks have their own page. A foundation crack can come from ordinary settlement, from expansive clay, or from loss of support under one corner. Sinkhole country adds a specific cause and a specific insurance product. Do not import Florida panic into a clay-soil suburb, and do not ignore a growing depression because the house is "not in Florida." Karst exists in parts of Tennessee, Kentucky, Pennsylvania, Texas, Missouri, and elsewhere. The map is public. The lot is specific.

What it is

Dissolution of rock below the soil, sometimes sudden, more often a slow raveling that shows up as a depression, doors out of square, and cracks that reopen after they are patched. A leaking water or sewer line can trigger a collapse by washing soil into an existing void. That is why a sinkhole investigation and a plumbing test sometimes belong in the same week.

How to spot it in the listing

Start with the state geological survey's karst or sinkhole susceptibility map, not with the listing remarks. Remarks will not say "sinkhole" unless a claim was paid and someone is proud of the repair, or a disclosure statute forces the word. Florida has specific sinkhole disclosure rules. Other states vary. Research beyond the listing means the survey map, the county property appraiser, and any permit for foundation grouting or underpinning.

Photos: a round depression in the yard, a fence that leans in one bay, pavement that has dropped at a joint, and interior cracks that were patched and cracked again in a later photo. One hairline in a slab-on-grade garage is extremely common and is not a sinkhole. A depression you can see on the aerial, centered near the house, is a question you ask before you offer.

How to spot it at the showing

Walk the yard, not just the rooms. Look for dips, exposed roots on one side of a tree, and downspouts that discharge into a low spot against the foundation. Inside, doors that were planed, cracks wider than a quarter inch, and patches in a floor that the seller calls "cosmetic" deserve a measurement. Ask directly whether there has been a sinkhole claim, a grout injection, or a foundation repair. In states with a disclosure form, read it. A "no" that is false is a different legal problem than a "no" you never asked.

The real risk

Catastrophic collapse is rare. The expensive ordinary case is a house that needs underpinning or compaction grouting, may be hard to insure, and will be harder to sell. A leaking utility that is feeding the problem will keep feeding it after the cosmetic patch.

Insurance and lending

In Florida, insurers must offer catastrophic ground cover collapse, and sinkhole coverage beyond that has been restricted, expensive, or unavailable depending on the year and the carrier. A prior sinkhole claim can make the house nearly uninsurable or insurable only with a large sinkhole deductible. Other states: most standard policies cover sudden collapse in a limited way and exclude earth movement. Read the form. Do not assume "we have insurance" includes a void under the slab. Lenders require insurance. If the only available policy excludes the risk the geology suggests, you have a financing problem. Get the quote before the inspection contingency ends.

Typical cost ranges

Typical ranges, not quotes. Testing is the point of the contingency.

Geotechnical look, sometimes with ground-penetrating radar: often $1,500 to $5,000 for an initial investigation. Borings cost more.
Plumbing test to rule out a leak washing soil away: often $300 to $800.
Compaction grouting or underpinning when a void or loss of support is confirmed: commonly $10,000 to $50,000, and serious sinkhole remediation can run well beyond that.
Cosmetic crack repair with no investigation: a few hundred dollars, and it does not answer the question.

Pass, negotiate, or investigate

Investigate when the map, the yard, or a disclosure puts the house in play. Do not investigate every slab crack in a non-karst suburb as if it were a sinkhole. That is how you waste the contingency. Negotiate when an engineer has a repair scope and the insurer has said, in writing, what they will still cover. Pass when there is a prior sinkhole claim, no engineering closeout, and your agent cannot find a policy that a lender will accept. The red-flag guide is for after the engineer writes, not before you have looked at the state map.

Sinkholes: Karst Maps, Floor Cracks, and the Insurance Endorsement