A land mine was an explosive device triggered by pressure, usually when it was stepped on.
While on Skaro near the end of the Thousand Year War, the Fourth Doctor stepped on a land mine and was forced to stay still as he expected it to detonate if he took his foot away. Harry Sullivan placed rocks beneath it to steady it and the Doctor moved slowly off it. (TV: Genesis of the Daleks)
In the 19th century, near Killingworth, the Rani hid in a wood special landmines, able to transform animal beings in vegetable beings. They were supposed to hit the Sixth Doctor, but eventually its victims were humans like Luke Ward, who was transformed in a near-sentient tree. (TV: The Mark of the Rani)