@prefix skos: <http://www.w3.org/2004/02/skos/core#> .

<http://www.opengis.net/def/nil/OGC/0/unknown> a skos:Collection,
        skos:Concept ;
    skos:definition "The correct value is not known to, or not computable by, the sender of this data. However, the correct value probably exists"@en ;
    skos:exactMatch <http://www.opengis.net/def/nil/ogc/0/unknown>,
        <http://www.opengis.net/def/waterml/2.0/medium/Unknown> ;
    skos:prefLabel "unknown"@en .