You create a hidden mine, an orb of invisible arcane energy which floats gently above the ground until something touches it, which causes it to explode. Two arcane mines can never occupy the same space, if two mines touch, they both explode. You cannot float arcane mines in midair, 5 ft. or more above a supporting surface (unless you use another spell or device to do so).

When you cast Arcane Mine, each creature which can see the mine’s location gets to try a DC 15 test of Seeking or Spellcraft to detect the mine.
A creature which is not aware of the mine must pass a DC 15 test of Seeking, Spellcraft, or use some other means for the mine to be revealed to them.
You may choose to make your mines plainly visible as a fast action.

Mines placed in an occupied space detonate instantly.
If a creature enters a space that has a mine in it, the mine explodes, and interrupts their movement. It can choose to stop or keep moving after the explosion.

When a mine explodes, each creature in a 15 ft. cube centered on the mine gets a health (reflex) save. Inanimate objects automatically fail.

Fail:
The target takes 1d8 elemental damage.
Intensify: +1d8 elemental damage per boost from Intensify.

Pass:
The target takes half as much damage from the mine.

Mines only explode once and are destroyed in the process.
As an action, with a snap of your fingers, you can remotely detonate any of your mines that you want to. They will normally explode simultaneously.

Arcane mines are quite unstable. Touching them is not the only thing that can set them off. If they are disturbed enough, exposed to sufficient trauma (such as an attack), a high concentration of arcane energy (including another mine’s explosion), or mundane fire or lightning, they may explode.

If multiple arcane mines explode simultaneously (ex: multiple mines are caught in a blast, setting them all off), for each affected target, combine the damage that the mines deal and treat it as one instance of damage. (Abilities which mitigate one instance of damage should mitigate the damage from one large explosion effectively.)
Subsequent stages of a chain reaction of explosions should still be treated as separate.
(ex: Triggering a mine by stepping on it would be the first explosion, then all the explosions of the mines caught in that explosion should be resolved together, then all the explosions of mines caught in that explosion, and so on.)
Variant Rule: For expediency, the Headmaster may decide to just determine all of the mines that should explode as a result of any chain reactions, and resolve all the explosions as happening simultaneously.

While Arcane Mines is a hostile ability, it is subtle, and placing Arcane Mines does not break Invisibility or cause its caster to become unhidden. The caster remotely detonating them does.

Subtle:
Casting this spell while hidden also hides that you are casting it, and does not cause you to become unhidden.
When you cast this spell while you are not hidden, you can try to hide that you are casting it with a contest of Spellcraft or Roguery against the Seeking of creatures observing you. Only observers that win notice you are casting a spell. However, once the spell is cast, its effects may be noticed.

Suggested Backfire: The mine appears on a space of the Headmaster’s choice, and you detonate all of your Arcane Mines.

Affinity:
When you take this class, choose which element your Arcane Mines are.
Whenever you cast this spell, it will normally be of that element.
You can try to cast this spell using a different element, but must pass a DC 20 test of Spellcraft each time to change the element from what it normally is.

Incompatible: Dilute and Concentrate from Advanced Geometry.