Since version 102.2 of the game, the way to build underwater tunnels has changed, because Underground slope was implemented. After several attempts (with multiple failures ;D ) and reading some post on the forum about how works the new underground slopes, I wrote this tutorial - how to build underwater tunnels.
1) Choose the 'vertically raise/lower land' tool, select the 'lower square' tool, and dig a pit of 2 squares length on each side of the land.
(http://img214.imageshack.us/img214/7835/tunel01128eng.jpg)
2) From the tool 'vertically raise/lower land' select the 'raise square' tool and create a slope on each tile on both sides of the ocean.
(http://img21.imageshack.us/img21/9831/tunel02128eng.jpg)
3) From the 'vertically raise/lower land' tool select the vertical artificial slope tool, build two artificial slopes and lay the road (or railroad) as shown in the picture.
(http://img3.imageshack.us/img3/8466/tunel003eng.jpg)
4) From the 'Road tool' select the 'Road tunnel' tool and then press Ctrl + click on the artificial slope for the tunnel entrance (only the tunnel entry is built, not the whole tunnel). Repeat on the other tunnel entrance.
(http://img696.imageshack.us/img696/6173/tunel005.jpg)
5) Open the 'Game Options' (diskette icon), select 'Display', then check 'sliced underground mode' and clicking on the tiny arrows choose the value -2
(http://img81.imageshack.us/img81/8040/tunel006eng.jpg)
In the underground view can be seen the entrance of each section of the tunnel at water level.
(http://img688.imageshack.us/img688/143/tunel007.jpg)
6) Now we push down the road 1 level, so the tunnel will pass under the water. From the 'vertically raise/lower land' tool select the 'lower artificial slope' tool and click at the road-end of each section of tunnel. As a result we have sent the road one level below.
(http://img514.imageshack.us/img514/6482/tunel008.jpg)
The road was lowered 1 level underground
(http://img403.imageshack.us/img403/1016/tunel009.jpg)
7) In the 'sliced underground mode' click on the arrow and set the underground level to -3. You can see the 2 sections of road that we have sent one level down. Select the 'Road tunnel' tool, then click on the start tile, then the end and a tunnel will be built between these points. Uncheck 'sliced underground mode'.
(http://img214.imageshack.us/img214/3435/tunel010.jpg)
(http://img187.imageshack.us/img187/845/tunel011.jpg)
8.) From the tool 'vertically raise/lower land' select the 'Restore natural slope' tool and remove the artificial slopes built in 2)
(http://img81.imageshack.us/img81/4903/tunel012.jpg)
The tunnel finished:
(http://img525.imageshack.us/img525/2729/tunel013.jpg)
This tutorial in german http://www.simutrans-forum.de/forum/thread.php?threadid=4976
Oooh! I hadn't figured it out yet. Awesome! I'm very very thankful for this.
Slightly offtopic- I think it's cool that you can now raise/lower end-road tiles, even above ground. :D
Split off unrelated discussion here:
http://forum.simutrans.com/index.php?topic=3842.0
Just to try and keep this thread more on-topic, and free up further discussion there if desired.
Well the tutorial is also on the German wiki (in German and Swedish), see f ex http://simutrans-germany.com/wiki/wiki/tiki-index.php?page=de_tutorial_UnterWasserTunnel
The Simutrans Reference manual and Simutrans Starter Guide should be updated with a good description of the new features and changes to the existing one.
The new underground slopes, sliced underground mode and new way of building underwater tunnels (from 102.2) should be added there (possible as an addition to the existing description as it is for older versions).
Diegotes description is quite and pretty good and ilustrative, but something like this should be put on one common place, where such features are described...
Hope you dont mind me posting into such an old post. I couldn't find this guidance in the manual and despite its age this post seems to be the latest on the subject.
I share the same sentiments of Isaac who ages ago said ..'I'm very very thankful for this'. It just helped me to work out how to build an underground slope, curve and station. (V111 Pak128)