There is a land unit type "spy". These can be very valuable land units to have wandering around the planet...
Spies are very special land units. They are able to march into any sector (owned or unowned) and look about. Of course, if you are marching in unallied territory, there is a chance you will get caught.
If a spy is caught by a friendly or neutral country, the country will get a message that a spy was spotted in their country. If the spy is caught by a hostile or at war nation, the spy gets shot on the spot. The chance of getting caught while marching about is inverse to the efficiency of the spy. So, at 100%, a spy has a 10% chance of getting caught. at 10%, a spy has a 100% chance of getting caught. Thus, you want efficient spies.
Spies are also very fragile. If they are caught in any kind of combat, shelling, bombing, etc. and take ANY damage at all, they die.
Spies may also be snuck on shore via ships using the "assault" command. If you assault a sector using just spies, they have a 10% chance of making it on shore without being spotted (no matter what their efficiency). If spotted, the same rules apply for how a spy is dealt with. If you mix spies and mil, the spies must fight just like normal units. And, since any damage they take kills them, this is very risky usage of spies.
You may locate spies using the "llookout" or the "spy" command.
Use your imagination on how to eradicate errant spies. ;-)
Submarines may carry up to two spy units.
When unloaded from a ship, spies are not given over to the owner of the sector as other units are.
See also : lboard , bomb , fire , spy , llookout , LandUnits