The Air obelisk is located in level 7 Wilderness, and can be reached by climbing the ladder north of the Black demons in Edgeville Dungeon. It is used for enchanting unpowered orbs into air orbs (requires level 66 Magic), which costs 3 cosmic runes and 30 air runes per orb charge. This gives 76 Magic experience. At level 66 Crafting, an air orb can be attached to a battlestaff to make an air battlestaff, granting 137.5 Crafting experience.
As the Air obelisk is in the Wilderness, extreme caution should be taken when visiting it due to player killing, and an unprepared charger can end up losing their orbs and runes. Bringing teleport items such as an amulet of glory or runes may be wise, although one still has to be careful, since players with a high Magic level may teleblock you.
Enchanting orbs into air orbs is a good way of training Magic as one can earn up to 40,000 experience an hour.
The most efficient way of enchanting large numbers of air orbs is to run to the obelisk from Edgeville bank and teleport back using a charged amulet of glory, allowing fast banking and thus faster training.