Where are the land mines?

[-=9315+35/<r4/VX=-]

Hi, I am a little bit confused about the land mines. There is a cooldown for land mines In the resources tab. But I can't find any land mines at all. Where do I get or use them?

[Demimind]

The landmines are those "last line" explosives in the doorways. The first zombie that hits it and the nearest ones in a tight group are killed in the initial explosion, but any zombies that get past after that before they recharge will take lives.