It's hard to apply the adjective "cool" to machines of war, but this 100-year-old photo is fascinating. It shows a number of German U-boats in port around 1915. Click on the image for a more detailed version.
Source: Library of Congress Also, don't forget to check out this great photo of the Lusitania.
Photograph shows German submarines in a harbor. Front row (left to right): U-22, U-20 (sank the Lusitania), U-19 and U-21. Back row (left to right): U-14, U-10 and U-12.