What is big bertha in ww1?

Early in WWI (1914) Krupp built huge siege cannon know as "Big Bertha" to flatten defensive forts during the invasion of Belgium.

These monsters fired a 16.5 inch projectile 8 to 9 miles. They were fused to explode, not on initial contact with the target thus allowing the explosive projectile to penetrate the fort's defensive barriers and explode in the interior.

The guns weighed about 75 tons each and were transported on rail cars.