Every buried item shrinks toward the soil at a steady rate set by its own decomposition time — the typical years it takes microbes, oxygen, moisture and sunlight to break it down into simpler compounds:
progress(t) = min(1, age / lifetime)
scale(t) = 1 − progress(t)
depth(t) = progress(t) × sinkDepth
This is a simplified linear model of a real biological/chemical process that is usually closer to exponential and depends heavily on local conditions — the values used here (banana peel ≈ 1 month, paper ≈ 3 months, an aluminum can ≈ 200 years, a plastic bottle ≈ 450 years, a glass bottle effectively never within a human lifetime) are the commonly cited midpoints from environmental science references.
- Time speed — how many simulated years pass per real second, so you can watch a banana peel vanish in seconds while a plastic bottle still needs a slider push to millennia.
- Sinking — as an item decomposes it visually settles into the soil, standing in for it turning into soil nutrients (organics) or simply degrading in place (plastics/metals/glass).
- Fully decomposed counter — ticks up the moment an item's progress reaches 100%; glass and some plastics may never reach it during a realistic session, which is itself the point.
Real-world relevance: this is the same reasoning behind "reduce, reuse, recycle" campaigns and landfill design — knowing which materials return to the earth in weeks versus which persist for centuries drives decisions about composting, recycling and packaging bans.