I hope you're a patient guy
1. Insert > choose "Indestructable BHD with 2 miniguns" > hit Insert again to exit.
2. Edit > Groups > select #3 and call it "heli" (or whatever you like) > click "OK" to exit.
3. Click the heli to get the info in the panel > click the panel to bring up the Edit items Attributes window > change "None" in "Group" to #3 heli > click "OK" to exit > hit "U" to deselect.
4. Edit > Waypoints > click #1 and type "heli" as the description > OK (to exit).
5. Insert > Markers > click the top item in the list, then hit "W" to take you to "Waypoints" (saves all that scrolling) > put at 12 WPs on the map in whatever pattern you want, but make sure the last one is placed at the starting point.
6. Right click the first WP you placed on the map and choose "Select this type" from the menu to highlight them all > click the info panel to bring up the Edit Item Attributes window > change "List" from "None" to "#1 - heli" > click OK to exit. All the WPs should now be joined together. Hit "U" to deselect.
7. Edit > Area Trigger > click #1 and call it "heli" > "OK" to exit.
8. Insert > Area Trigger > place two of them diagonally opposite one another around the heli > right click one of them, choose "Select this type" from the menu > click the info panel to get the Edit Item Attributes window and change "#0" to "#1" under Waypoints (the dropdown menu will be disabled btw). Click OK to exit > hit "U" to deselect. Repeat steps 7 and 8, but put the second trigger at Waypoint #7 and call it #2 in the Area Trigger list.
9. Edit > Events > change "Delay" from "0" to "40" (without quotes) > double click "New Trigger", change Trigger Type to "Group" > Trigger Condition to "Area Trigger" > under Trigger Variables, change Group to "3- heli" and Area to "1- heli". it should now say "Group 3 is within area 1" under "IF".
10. Double click "New Action" under "THEN" > change Action Type to "Redirect Group" > Group to "3- heli" > WP list to "1-heli" and leave WP number at "0". It should now say "Redirect group 3 to waypoint 1 - 0".
11. Double click "New Action" under "THEN" again > under "Action type" choose "Change AI, Group" > under "Action subtype" choose "Combat speed" > under "Group" choose "3- heli" > under "Speed" type 25 (or scroll down and choose 25 from the list if you prefer).
12. Click "New event" in the left hand window to change it to "Event 1" > double click "New Trigger" > underr "Trigger type" choose "Group" > under "Trigger Condition" choose "Reached Waypoint" > under "Group 1" choose "#3 - heli" > under WP list, choose "#1 - heli" > under WP number choose "3" > click OK.
13. Double click "New Action" under "THEN" again > under "Action type" choose "Change AI, Group" > under "Action subtype" choose "Patrol Speed" > under Group choose "3- heli" > under "speed" type 15.
14. Click "New event" in the left hand window to change it to "Event 2" > double click "New Trigger" > under "Trigger type" choose "Group" > under "Trigger Condition" choose "Reached Waypoint" > under "Group 1" choose "#3 - heli" > under WP list, choose "#1 - heli" > under WP number choose "7" > click OK.
15. Double click "New Action" under "THEN" again > under "Action type" choose "Change AI, Group" > under "Action subtype" choose "Patrol Speed" > under Group choose "3- heli" > under "Speed", leave it at "0".
16. Double click "New Action" under "THEN" again > under "Action type" choose "Change AI, Group" > under "Action subtype" choose "AI State" > under Group, choose "3- heli" > under "Ai State" choose "Land".
17. Click "New Event" to change it to "Event 3" and under "IF" double click "New Trigger > change Trigger Type to "Group" > Trigger Condition to "Area Trigger" > change "Group" to "3- heli" > change "Area" to "2 - heli" > click OK.
18. Double click "New Action" under "THEN" once more > under "Action type" choose "Redirect group" > under "Group" choose "3 - heli" > under WP list, choose "1 - heli" > under WP number, choose "8".
19. Double click "New Action" under "THEN" once again > under "Action type" choose "Change AI, Group" > under Action subtype, choose "AI State" > under Group, choose "3 - heli" > under "AI State" choose "Follow Waypoint".
20. Double click "New Action" under "THEN" again > choose "AI State" > "Patrol Speed" > "3- heli" > "20" (or whatever speed you want - I presume you know the menu setup by now too).
21. New Event (will be Event 4) > New Trigger > Trigger type = Group > Trigger Condition = Reached Waypoint > Group = "3- heli" > WP list = #1 > WP number = 12 (assuming you put 12 WPs on the map).
22. New Action > Change AI, Group > Patrol Speed > "3 - heli" > Speed = 0.
23. New Action > Change AI, Group > AI State > "3 - heli" > Land.
24. New Event (Event 5) > New Trigger > Group > Area Trigger > "3 - heli" > Area = "1 - heli" > OK.
25. New Action > Reset Event 0.
Should work
Oh, and you can't "dry out" the tunnels. If you want tunnels and water, use DVDG2 as your terrain and build your map on the plateau with the sea at the front and rivers on either side. The plateau is elevated sufficiently to allow you to place both tunnels and water without flooding the tunnels.