Sinkhole Alley by the numbers: what Hillsborough, Pasco and Hernando reported

Hillsborough, Pasco and Hernando are the three counties our guide to Florida’s sinkhole-prone areas names as the heart of Sinkhole Alley. Between them they hold 1,265 of the 4,417 reported subsidence incidents in the state’s record, more than a quarter of the whole. This page counts those 1,265 reports: when they came, who filed them, how big the reported holes were, and how many were ever confirmed as sinkholes. The short answer to the last question is 46.

What a report in this record is

The Florida Geological Survey keeps the statewide database of subsidence incident reports. Each row is one report that the ground sank somewhere, filed by a resident, a county office, a research program or an emergency call. Most were never checked by a geologist. The Survey’s own description of the data puts it this way:

“While this data may include some true sinkholes, the majority of the incidents have not been field-checked and the cause of subsidence is not verified.”

So a report is a reported subsidence incident, and this page calls it that. Only a row the Survey marks as verified is counted here as a sinkhole.

How we counted

We counted from the state’s download of September 27, 2026, whose latest report is dated May 2026. One row is one report. A report belongs to a county by the record’s county field and to a year by its event date. 36 of the 1,265 reports carry no event date: they count in every total but in no year. Sizes come only from reports that give a positive width or depth; two entries of 999, which read as a placeholder rather than a measurement, are left out.

Sinkhole Alley's reports came in bursts

Year by year, the three counties’ reports are not a steady stream. Six bursts, none longer than a month, hold 298 of the 1,265 reports, nearly a quarter of them.

1964. The 45 Hillsborough reports dated May 1, 1964 come from one study, not from 45 calls. Each carries the same date and the same comment, pointing to a U.S. Geological Survey report by Bill Sinclair on sinkhole development resulting from groundwater withdrawal. Read the date as the record’s, not as a day when 45 holes opened.

1977 and 1985. Both Hillsborough bursts point to freeze protection, water pumped onto crops to guard them against frost. Of the 30 reports from January 1985, 25 note that freeze-protection pumping was under way at the time, and 22 share a single date, January 21. The 22 reports of January 20, 1977 all send the reader to those 1985 reports for later information.

2001. Hernando County’s July 2001 burst is the one the record ties to dry weather. 22 of its 26 reports note extreme drought conditions, several of them at stormwater ponds.

2010. The largest burst in the record: 137 Hillsborough reports in January 2010, 109 of them in the five days from January 11 to 15. They came through calls to the Southwest Florida Water Management District, the City of Plant City and a well construction office in Brooksville, among others. The record gives no cause for them, and none was verified as a sinkhole.

2012. Ten days in Hernando County produced 38 reports, all from the county’s emergency management office, and 35 of them are marked as verified sinkholes. That is every one of Hernando’s verified sinkholes but one, and a third of the 103 verified sinkholes in the whole state’s record. Several sit on the edge of drainage retention areas.

County by county, decade by decade

The three counties do not share a timeline. Hillsborough’s reports gather at both ends, in the 1960s to 1980s and again in the 2010s. Pasco’s are a 1980s story: 140 of its 339 reports fall between 1985 and 1989. Hernando’s peak is the 1990s, with 100 reports.

Who filed the reports

Nearly half of the three counties’ reports, 615 of 1,265, credit the Florida Sinkhole Research Institute, with event dates from 1964 to 2004. The Institute accounts for 358 of the 374 reports dated in the 1980s, the decade that looks busiest in Pasco and Hillsborough. From 2005 on, the reports come from many hands: calls to the water management district, county emergency managers, fire departments and public works offices, cities and residents.

That matters when you read the chart. A tall bar records how many reports reached the database, which depends on who was collecting and when. It is not a measure of how much ground gave way that year.

How big the reported holes were

Most were small. Of the 849 reports that give a width, half are 6 feet across or less, and 599 are 10 feet or less; 28 are 50 feet or more. Of the 764 that give a depth, half are 5 feet deep or less, and 77 are 20 feet or more. The widest is 200 feet, given twice: a Hernando County report from 1974 and a Pasco County report from August 2017 whose comment records three homes condemned. The deepest given is 100 feet, two Hernando reports from 1974 and 1998.

314 of the 1,265 reports record damage to property. 371 record none, and the rest do not say.

How many were verified

Of the 1,265 reports, 46 are marked as verified sinkholes: 36 in Hernando, 7 in Pasco and 3 in Hillsborough. Another 26 were checked and found not to be sinkholes. The other 1,193 were never confirmed either way. Hillsborough, with more reports than any county in Florida, has 3 verified sinkholes in the record.

None of this makes the ground in these counties safe. Our page on why Florida has so many sinkholes explains the limestone and clay that put them at risk. What the record shows is narrower: a report is a call about sinking ground, and the count of calls follows the people making them as much as the rock.

For the other end of the state’s range, our page on sinkholes in St. Lucie County goes through that county’s three reports, none verified.

Sources

Every count on this page is ours, made from the Florida Department of Environmental Protection, Florida Geological Survey, Florida Subsidence Incident Reports, the state’s open data, downloaded September 27, 2026. The quotation is from the Survey’s description of the dataset.