TL;DR
When you're dragging a belt, if you drag over an obstacle, an underground belt is automatically added. If the obstacle is too large, you get a warning sound and a 'gap too large' message. But if you drag over two obstacles which have a one-tile gap between them that are collectively too large for an underground, the system will fail without a warning.What?
If you are dragging a belt:- If you pass over an unbuildable area or obstacle which is small enough to bridge with an underground belt of the same type, such an underground is automatically added. This happens when your mouse reaches a buildable space on the far side of the obstacle, with the underground entrance retroactively replacing the section of ordinary belt that was place before it and an underground exit is added in the tile you are pointing at.
- If the obstacle is too long to span, you get a loud 'BLERK' warning sound and a written 'gap too large' message floating over the offending location. Neither the underground entrance nor exit is created, with both tiles being left as ordinary belts.
- If you pass over an obstacle, then a single buildable tile, then another obstacle, and the collective size of both obstacles (and the gap between them) is small enough to bridge, then a single underground belt spanning both obstacle is added. This happens when your mouse reaches a buildable space on the far side of the second obstacle, with the underground belt exit which was placed in the gap when you passed over it being removed and a new exit is placed on this space instead.
- If you pass over an obstacle, then a single buildable tile, then another obstacle, and the collective size of both obstacles and the gap between them is too long to bridge, then the second obstacle will not be bridged, yet no warning of this failure is given. Instead, the first underground belt, spanning the first obstacle, is left in place, with the underground exit in the one-tile gap, and an ordinary belt is built on the space after the second obstacle. The engineer continues on his merry way, unaware that the belt he has just built is broken.