Mont Blanc

Mont Blanc (モンブラン) is the name used to refer to special bombs used by England. They are first introduced in the 8th Shinyaku Toaru Majutsu no Index light novel, and was shown being used on the Sargasso of the North Sea near Iceland during the Anti-GREMLIN Alliance attack there along with the Grape Jelly bombs.

The weapon was nicknamed after a Mont Blanc, a chestnut dessert.

Principles
The mechanics of the bomb are not entirely known. What's known is that it uses explosions to drive thousands or even tens of thousands of sharp metal stakes to rain down upon the target.