Geological Appraiser

If you can't cast the discovered card (perhaps because there are no legal targets for the spell), you'll put it into your hand.

Geological Appraiser's triggered ability triggers if you cast it from any zone. It doesn't trigger if you put Geological Appraiser onto the battlefield without casting it.

You exile the cards face up. All players will be able to see them.

If you cast a spell "without paying its mana cost", you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the spell has any mandatory additional costs, you must pay those to cast it.

A spell's mana value is determined only by its mana cost. Ignore any alternative costs, additional costs, cost increases, or cost reductions.

The mana value of a split card is determined by the combined mana cost of its two halves. If discover allows you to cast a split card, you may cast either half (as long as its mana value is less than or equal to the effect's discover value) but not both halves.