SinkholeOdds

Sinkhole risk in Miami-Dade County, FL: Sinkhole country

Sinkhole country

Soluble rock at or near the surface across much of the county.

On soluble rock
92%
At or near the surface
85%
Buried under cover
7%

What to do about a house in Miami-Dade County

Get a sinkhole or geotechnical inspection before buying, building or adding weight to a foundation.

Standard Florida homeowners policies cover only catastrophic ground cover collapse; full sinkhole coverage is a separately priced option insurers must offer.

That works out to roughly 1,742 of Miami-Dade County's 1,900 square miles on soluble rock, 1,613 of them with the rock at or near the surface. With about 2,838,461 residents (1,494 per square mile), there is effectively nowhere in the county off the soluble rock, town or country. The figures come from 312 map points about 2.5 miles apart, so a pocket smaller than that can be missed or overstated.

Two things to read before deciding: what Florida insurers have to cover, and why gypsum and salt move faster than limestone.

Counties around Miami-Dade County

The closest county with no soluble rock mapped is Gulf County, FL, about 413 miles northwest. 5 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

What Miami-Dade County is built on

The largest mapped soluble unit here is the Miami Limestone (limestone), Pleistocene in age, covering about 83% of the county. Next is the Miami Limestone at 7%.

The county also lies over the Floridan shelf, a deep salt or gypsum basin. Deep beds rarely cause surface collapse on their own, but wells and mining into them have.

Against the rest of Florida

Miami-Dade County ranks 9 of 67 in Florida for soluble rock at the surface, and sits above 95% of US counties on that measure.