Route your belts freely underground — around corners and over any distance — without ever popping back up to the surface.
Beltlayer gives you a dedicated underground layer for your belts. Drop two connectors, step into the underground editor, and run belts between them however you like. No more straight-line-only underground belts, no more surfacing every few tiles, no more weaving spaghetti across the surface just to cross a bus.
This is a maintained continuation of the original Beltlayer by Therax, ported to Factorio 2.0 / Space Age and extended with bug fixes, full Factorissimo 3 support, and undergrounds that mirror the world above.

How it works

Build connectors. Beltlayer connectors are unlocked alongside the normal underground belts of the same speed, and share the same recipe — no extra research grind.
Place a pair. Drop one connector where items go down and one where they come back up. Any rotation works; switch a connector between input and output with the rotate key (default R).
Go under. Press CONTROL + B (rebindable) to drop into the underground editor. You'll see the two connector endpoints, and any belts or underground belts from your inventory come with you.
Lay belts freely. Route belts between the endpoints using the exact belt mechanics you already know — corners, merges, the works.
Come back up. Press CONTROL + B again to return to your character on the surface. Done.

Features
Factorio 2.0 & Space Age ready. Works on every planet. Connectors are generated automatically from whatever underground belts exist in your install — vanilla, Space Age, or modded belt tiers.
Engine-native, UPS-friendly. Connectors use Factorio's built-in linked-belt mechanic, so items move across the surface/underground boundary at the engine level with zero per-tick scripting. The mod is fully event-driven — there are no polling loops scanning your base every tick. The only periodic work happens while a player is actually standing in an underground, so the cost scales with players underground, not with the size of your factory.
Factorissimo 3 support. Use Beltlayer inside your factory buildings. Each top-level factory gets its own underground, confined to the exact size of the building's interior and bordered by clear black void so you always know where you can build. (By design, nested factories don't get their own underground — one build area per factory keeps things balanced and UPS-light.)
Undergrounds that match the surface. Planet undergrounds mirror the tiles from the world directly overhead, so going underground feels like digging straight down. Water is flattened into matching ground, and trees, cliffs, and decoratives are left out for a clean, flat workspace. Whatever tileset your surface uses — vanilla terrain or Alien Biomes — is reproduced automatically, with no setup required.
Blueprintable & bot-buildable. Beltlayer connectors and their underground belts can be captured in blueprints and built by construction robots.

Compatibility

Space Age — fully supported, including non-Nauvis planets.
Factorissimo 3 (factorissimo-2-notnotmelon) — optional; enables in-factory undergrounds.
Alien Biomes — optional; its tiles are mirrored into planet undergrounds automatically.
Designed to play nicely with belt-overhaul mods (Bob's, and others) by generating connectors from their underground belts.

A note on balance
Yes, this is a little cheaty — the straight-line and length limits of vanilla underground belts are part of Factorio's design challenge. But you still have to move items on the surface to feed machines and circuits, and keeping track of routing you can't see from above adds its own kind of complexity. Use responsibly, or don't. Your spaghetti, your rules.
Known limitations

Blueprinting or cloning a Factorissimo factory does not yet copy its underground contents.
Blueprint and deconstruction handling across the surface/underground boundary is intricate. If you hit a bug, please report exactly what you did — ideally with a screenshot of the selected area, a screenshot of the underground editor for the same spot, any deconstruction-tool filter settings, the blueprint setup window if relevant, and what you expected versus what happened.

Bugs & feedback
Found a bug or have a suggestion? Please post it on the Discussion tab of this mod's portal page rather than in the comments — it's much easier to track there. A good report includes your Factorio version, the mods you're running (Factorissimo, Alien Biomes, belt overhauls, etc.), clear steps to reproduce, and screenshots where they help. The more precisely you describe what you did leading up to the problem, the faster it can be fixed.

Credits

Therax — original Beltlayer.
Orillien — Factorio 2.0 / Space Age port.
Lux — this continuation: bug fixes, Factorissimo 3 support, and mirrored underground terrain.