This plain, iron spike feels warm to the touch and pulses with stored energy.

An exploding spike becomes invisible 1 round after you plant it firmly in the ground. Thereafter, as soon as any creature comes within 10 feet of it, the spike explodes in a fireball that deals 10d6 points of fire damage (Reflex DC 14 half) to every creature within 20 feet of it.
This blast destroys the spike. During the round that an exploding spike remains visible after being planted, any creature can safely remove it from the ground (a standard action) to render it harmless and ready for reuse.

Prerequisites: Craft Wondrous Item, fireball.
Cost to Create: 500 gp (plus 500 gp for base item), 40 XP, 1 day.