Spike Trap and Cast into the Underworld

Spike Trap reads: "... At the start of the environment turn, deal the target with the lowest HP 3 melee damage."

First question: Who deals the damage? This card?

Cast into the Underworld read: "Increase damage dealth by environment targets by 1..."

Second question: Spike Trap is not a target (it has no HP), so it's damage will not be increased?

Typically, if not stated, it's assumed that the card is the source of damage.  And you are correct!  Since it is not a target the damage is not increased.