Underground belts

Items

belt-type	factor speed	speed (tiles/sec)
File:Basic-transport-belt-to-ground.png Basic underground belt	1	1.875
File:Fast-transport-belt-to-ground.png Fast underground belt	2	3.75
File:Express-transport-belt-to-ground.png Express underground belt	3	5.625

Task: you need to cross another belt, pipe, or any other entity; not more than 5 tiles.

select underground belt and make sure, it is in the right direction (press R until it is)
place the to entry-section
the underground belt keeps selected but it has changed into the exit section
place it in the direction of the entry, not too far away.

Note: Rotation key (R) can also be used to invert tunnel flow of placed underground belt.

For a more founded introduction into the belt mechanics see Belt transport system.

Underground belts can be used to cross different flows of items without interfering. They move items like a normal belt.

You can turn the order of the lanes or split the lanes completely with the underground belts trick

You can cross any number of entities and all types of ground, like water, swamps, etc. (input and output of course over water)
You can cross other underground stuff (any number of underground belts or underground pipes). They won't be mixed.
For the connection only the endpoints (entry-side and exit-side) are relevant.
The maximum distance underground is 5 tiles.
An underground belt pair with a blocked output stores up to 20 items. (Per lane? Untested!)
If you mine an underground belt, up to 20 items are recovered, the remaining items in the underground are placed into your inventory.
The half of the underground belt tile with a belt can accept input from the side. The other half (with a tunnel entrance) blocks incoming items.

This last fact is important, because it can be used to do some tricks.

This explains in detail the splitting lanes trick. This is a very useful "misuse" of the underground-belt output. See right!