Land mine

Harry Sullivan tries to steady a mine the Fourth Doctor has stepped on. (DW: Genesis of the Daleks)

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. (DW: Genesis of the Daleks)