Foiba (plural, foibe) is a term derived from the northeastern Italian karst region. The word is here suggested for use in preference to other terms referring to vertical cavities in soluble rocks. Foiba is defined as a cavity in relatively soluble rock which is natural, solutional, tends toward a cylindrical shape, and possesses walls which normally approach verticality. In laboratory experiments, limestone blocks were treated with dilute hydrochloric acid, and cavities resembling foibe were produced. Vertical walls developed only when a less soluble layer capped the limestone block or when the acid source was stationary, allowing acid to drip to the area directly below. Water analyses from foibe in central Kentucky and Missouri indicate that the water has had less residence time in the zone of aeration than other waters percolating through the rocks and entering the caves. In central Kentucky, foibe seem to be developed by migrating underground waterfalls held up by less soluble layers or by water moving directly down joints below less soluble layers. In Missouri, foibe are formed by joint enlargement below chert layers. Those foibe in the ceilings of caves are complicated by the enlargement of the lower part of the joints by cave streams during fluctuating water table conditions. In limestone caves of Kansas, foibe are formed in a similar manner as in Missouri. The foibe of the gypsum caves of Kansas are formed mainly on the sides of steep collapse sinkholes and lack joint control although they form beneath less soluble layers in the gypsum. Dripping water is necessary for the development of vertical walls by solution. Less soluble layers seem to be the unique feature which allows water to drip and pour into foibe. The floors of foibe are formed by less soluble layers or near the water table. If foibe intersect previously formed cave passages, no floors may develop.