Here's some interesting photos.

[Claven2]

Many of us know that the Germans used giant siege mortars in 1914 to reduce the Belgian fortresses of Liège during the early stages of the Schlieffen Plan.

Images of these monsters are rare, but here are some pics of the two types of guns used.

1) The Austrian built 305mm Skoda siege mortar:
http://www.worldwar1.com/foto/ah159b.jpg

2) The 420mm Krupp siege howitzer:

http://www.worldwar1.com/foto/tww201c.jpg

At the time, these were the largest land based guns in the world, though by the end of the war the French fielded the largest diameter artillery piece, a massive rail car mounted canon firing a 520mm shell.

[woodchopper]

they sure don't make them like that anymore.

however there are some big morters out there (210mm and bigger) that would be employed in the same roll.

However when you can call in an airstrike with a few 1000lb bombs who needs to lug around those big tubes.

[Claven2]

Well, in 1914 they didn;t have bunker busters and J-Dam's ;)