sky anchors
...that sky anchors combine a gas balloon for buoyancy and a superpressure balloon for ballast?
...that <b id="mwCCk"><a href="./Sky_anchor" id="mwCCo" rel="mw:WikiLink" title="Sky anchor">sky anchors</a></b> combine a <a href="./Gas_balloon" id="mwCCs" rel="mw:WikiLink" title="Gas balloon">gas balloon</a> for buoyancy and a <a href="./Superpressure_balloon" id="mwCCw" rel="mw:WikiLink" title="Superpressure balloon">superpressure balloon</a> for ballast?