Terraforming Mars: Prelude 2, the sixth expansion for Terraforming Mars, focuses on cross-expansion effects, prelude cards with ongoing effects and actions, and more project cards for your tableau...not to mention five new corporations.
Most of the cards are specifically connected to other expansions of Terraforming Mars (Venus, Colonies, Turmoil), but the 5 Corporations and 13 of the Prelude cards are playable without any of these other expansions.
