| UNCONFORMITY | A time break in a depositional sequence. Geology. Ref KG |
| UNSATURATED ZONE | In karst, the zone where voids in rock are partly filled with air and through which water descends under gravity. syn Vadose zone. Speleology term. Ref AC |
| UVALA | A complex closed depression with several lesser depressions or dolines within its rim. Karst - Depression Features of. Ref JJ |