New conversation with new people! :-)
Here's how I think landmines should work:

* landmines Labeled by color --- green or blue.

* Friendly players can clearly see their landmines, but start invisible to the enemy. Enemy players have a 33% chance per second to reveal a landmine's location (in a 10x10 area?) for all, just by simply ducking and having one in their field of view.

* Grenade-type damage to any enemy that steps on it.

* When detonated, blackens blocks and removes mines (friendly or not) in a 5x5 area. Therefore you can have several mines in a single area, but 25 placed mines will not equal 25 kills.

* Removable with a single hit (invisible or not), and can be removed by shooting, pickaxing, destroying the underlying block, or by building upon.

* Separate choice from grenades.

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I think landmines would be nice because they could add a certain fear-factor to always reusing the same tunnel to launch attacks. Tunnels made behind your intel are especially attractive to the enemy, since they're nearly impossible to control except to wall them off (a time-consuming process). Landmines would at least slow up players traveling them.
As for mines by the intel, it seems like a very obvious place to look for mines. With my suggestion, getting to a mine-carpeted intel should be easy task for the smart player. What do you think?