I would really appreciate details about BIG mines. I've never heard of such things. What would set one off? what was target? Did it take excavating equipment to place one? Forgive my ignorance, please and thanks!
The Battle of Messines was one of the most spectacular examples of sapper mining in WWI. There over a year's worth of planning, and tunnelling done in advance of the operation, and then they lit off some truly massive mines on the morning of June 7, 1917, killing upwards of 10,000 German troops in one fell swoop. The explosions could be heard in London.
The craters still exist today - most of them have filled up and become ponds. Google for "Mines of Messines" and you'll come across a lot of interesting photos, both historical and current.
Interesting side note:
They tunnelled in a total of 25 mines for the operation, but because it took a year to do it, by the time of the actual offensive the front line had shifted somewhat, and only 19 were lit off (the remaining 6 being too close to British/Anzac lines). Instead of removing the explosives, they just sealed off the tunnels and buried them in place. They have a general idea where they are, but not precise, and so there's been a lot of hesitation to go digging around looking for them. One of them was set off by a lightning strike in 1955.
As far as I know, the other 5 are still un-excavated, and there isn't a lot of research into the effects on the stability of Amatol after being buried in soggy mud for 100 years or so.