I was indulging in a little Wiki-wandering this morning when I came across an article about what must surely be the biggest mortar ever built.
It's the US Army's 'Little David', constructed for the invasion of Japan in 1945.
This incredible beast had a caliber of 914mm. (36 inches - a full yard!), and fired a shell weighing 3,650 pounds over a range of up to six miles.
It never saw action, as the dropping of the atomic bombs led to Japan's surrender without the need for an invasion; but it's still an awesome piece of engineering.
The sole surviving Little David is today on display at the US Army's Aberdeen Proving Grounds.
There's a lot more information about Little David in an article at Military Factory. Interesting reading.
Here's a video clip showing Little David in operation during trials in 1945.
Just looking at the amount of excavation and preparation needed before firing it indicates to me that Little David probably wouldn't have been very useful in action . . . but one never knows. If the invading forces had had enough time to set it up, and enough targets within its limited range once they'd done so, the beast might have justified its cost and complexity.