Downside Up

You fall into the sky.

Downside Up inverts gravity for one entity in the entire game: you. The
world generates vanilla, mobs walk the ground, sand and water and dropped items
fall down like they always have. But your body falls up. The surface of the
world is not a place to live anymore — it’s the edge of a bottomless pit, and
everything above it wants you dead.

So you live underground, upside down, walking on the ceilings of caves.

The start

Every new world spawns you deep in a lush cave, standing on its ceiling. It
looks strange for a minute and then it clicks: stone and ores are "under your
feet", the cave floor hangs overhead like a green sky, and glow berries dangle
right past your face.

You’re not dropped there to die, either. There’s always an azalea tree within a
few blocks of spawn and glow-berry vines around it — wood, light and food to
get you through the first day. Azalea trees also generate naturally on lush
cave floors everywhere else, and in low chambers their crowns reach the
ceilings you walk on. Wood is never a problem; reaching it safely is your
problem.

Falling — the wrong way

Step off a ledge and you don’t land somewhere below. You accelerate toward the
surface, shoot out of some ravine, and keep going — into open sky, with
nothing to grab, watching the world shrink under you.

The mod gives you a fighting chance, not a free pass:

• Above Y=320 the air thins out. You get Slow Falling: your momentum is
  cut immediately, the climb turns into a slow drift, and impact damage is off.
  But thin air is toxic — poison stings the whole time you’re up there. The
  poison can’t finish you off by itself; it just makes sure the trip is never
  free.
• Holding Jump while drifting dives you back toward the ground. That’s
  your rescue tool: aim for a waterfall.
• Above Y=800 the sky simply kills, at the same pace the void does. Nobody
  drifts forever.

Water runs upward

Water is the one thing in the world that’s on your side: it flows up (to a
generous height limit), so waterfalls climb cliffs and pour into the sky.
Any rising stream is a ladder, a brake and a lifeline at once — grab one
mid-fall and you live. Vertical swimming is three times faster than vanilla,
which turns water columns into proper elevators. A single bucket, poured at
the right moment, is the difference between a scare and a death screen.

Bubble columns work from the ceiling too, farmland hydrates from water below
it, so upside-down farms work the way you’d hope.

Everything else is mirrored properly

This is a physics rewrite, not a camera trick. Landing, fall damage, footstep
sounds, friction, honey and soul sand — all read from the block your feet
actually touch, i.e. the one above your head. You step over half-block ledges,
sneaking stops you from sliding off a ceiling edge, ladders catch you when you
fall upward, knockback throws you off the ceiling instead of squashing you
into it. Crouching resizes you from the ceiling down. The screen is flipped,
the mouse follows, and your character model honestly walks feet-up.

Hostile mobs get rebalanced around your altitude: nothing spawns in a
±16-block layer around you, density grows the higher (closer to the deadly
sky) you build, and your first day is completely peaceful — after that, the
game stops being polite.

Beds, respawning, the Nether, the End, portals and the Eye of Ender all work.
The game is fully beatable, start to credits, upside down.

Tuning

Everything lives in one config file (config/downside_up.json): the altitude
thresholds, sky damage, water rise limit, mob spawn layers, starting biome,
grace period — and a master switch that returns the game to vanilla. There’s
also a /downsideup operator command for diagnostics.

Good to know

• Requires Fabric API. Client + server; in multiplayer only players are
  inverted.
• Create a new world — the spawn point and starter pocket are built at
  world creation.
• Known rough edges (honest list): boats and minecarts stay vanilla, the
  Levitation effect still pulls skyward, swimming animations are vanilla, and
  the first-person hand is drawn in screen space, so it looks normal on the
  flipped screen.

Source code, full docs and issue tracker:
github.com/CloudWells/downside-up.
License CC0 — do what you want with it.