General The data source is a geospatial collection of polygons that correspond with the presence or absence of the Natura 2000 Annex I habitat type 8310 (Caves not open to the public) in the Flemish Region (and border areas), Belgium. The dataset contains all known, not collapsed, underground marl quarries in Flanders. Several of these quarries have their entrance in or run underground to the neighboring regions/countries. In general, different polygons represent different quarry units with their own internal climatic environment. Units that cross Flemish borders have been split into separate polygons. Exceptionally they may overlap if such units are situated above each other. For safety reasons, the dataset only contains the contour of the quarries, and no details like floor plans or entrances. For admission to research the indoor climate, please contact the Quarries and Safety Department of the municipality of Riemst ( https://www.riemst.be/nl/wonen/groeven ; mike.lahaye@riemst.be ). The data source is produced, owned and administered by the Research Institute for Nature and Forest (INBO, a scientific institute of the Flemish government). Technical aspects The data source is a GeoPackage that contains: a spatial polygon layer ‘<code>habitatquarries</code>’ in the Belgian Lambert 72 coordinate reference system (EPSG-code 31370 ); a non-spatial table ‘<code>extra_references</code>’ with site-specific bibliographic references. The data source has been based on an unpublished shapefile used in De Saeger & Lahaye (2019) and on a BibTeX bibliography file. See R-code in the GitHub repository 'n2khab-preprocessing' at commit c0821eb for the creation. A reading function to return <code>habitatquarries</code> (this data source) in a standardized way into the R environment is provided by the R-package n2khab . The attributes of the spatial polygon layer ‘<code>habitatquarries</code>’ are: <code>polygon_id</code>: a unique number per polygon; ...