Historical Rules · 15 February 2021
Rule 712.9
Official source text from the 15 February 2021 Rules release
If an effect allows a player to cast a meld card as a face-down spell, or if a meld card enters the battlefield face down, it will have the characteristics given to it by the rule or effect that caused it to be face down. That card remains hidden, using either a face-down substitute card or opaque sleeves. See rule 707, “Face-Down Spells and Permanents.”