An old, but sometimes still effective tactic...
http://www.heraldsun.com.au/news/world/dramatic-images-show-moment-syrian-rebels-use-tunnel-bomb-at-besieged-assad-army-base/story-fni0xs61-1226919742440
(https://www.grogheads.com/forums/proxy.php?request=http%3A%2F%2Fresources2.news.com.au%2Fimages%2F2014%2F05%2F16%2F1226919%2F744530-eb90d3e8-dc7d-11e3-bb5e-bb14174cd6c1.jpg&hash=5634ff0a81b2f83bc99d767bd3460efe9602b968)
...as a side note, I came across this story while checking out the above story: 50,000 lb WWI Bomb Found Under Belgian Farm (http://www.rense.com/general47/50000lbWW1bomb.htm)
Reminds me how the British used 22 mines and close to 500 tons of explosives to blow up Messines Ridge near Ypres in in 1917:
http://en.wikipedia.org/wiki/Messines_Ridge (http://en.wikipedia.org/wiki/Messines_Ridge)
I believe only 19 of the mines went off. One was discovered and destroyed by the Germans and the other two are still buried under the Belgian countryside somewhere...
It brought to mind The Battle of the Crater (http://en.wikipedia.org/wiki/Battle_of_the_Crater) during the American Civil War.
(https://www.grogheads.com/forums/proxy.php?request=http%3A%2F%2Foi57.tinypic.com%2Foity6o.jpg&hash=3dbb0304b6045533985598d3c777bf6145132425)
Wow. I wonder how long the Syrian rebels were working on that.
Those are some crazy pictures.