A wall integrity gauge in the dig view: the wall's picture cracks, crumbles and trembles as it gets closer to giving, from Stable to Failing.
Play with a friend: when you start the game, choose a public game anyone can join, friends only, or a private game you invite someone into.
Voice chat, using your s&box voice setting (push-to-talk is V).
Your partner's lantern burns teal, so you can tell whose light is whose from the far end of a tunnel, and their name shows over their head when you're close or they're talking.
A friend who joins comes in their own clothes.
Take the hit for the runner: if the boulder catches you while your partner is carrying the piece home, it slows as if it had smashed through a beam.
A co-op leaderboard page on the museum's board for museums finished together.
The museum says when a friend joins or leaves, and welcomes you when you join someone else's.
A microphone mark shows while your voice is going out.
If your partner leaves, the museum tells you, and anything they were holding goes back where it belongs.
Joining a friend's museum drops you straight in, without the introduction to a museum that isn't yours.
In a friend's museum, display cabinets, plinth enhancements and restorations are theirs to buy. Boosts are anyone's.
Loose rock in the tunnel ceiling could hang in mid-air under the roof after a new tunnel was built.
A museum you finish after playing any round with a friend goes on the co-op board, not the solo one.