Choose one or both — Destroy target artifact; and/or destroy target nonbasic land.
"Something down there has an appetite for what we're standing on. Let's hope it doesn't want seconds." —Samila, Murasa Expeditionary House
Official Rulings for Fissure Vent
6/15/2010 : You may choose just the first mode (targeting an artifact), just the second mode (targeting a nonbasic land), or both modes (targeting an artifact and a nonbasic land). You can't choose a mode unless there's a legal target for it.