"I cannot help but laugh at this unearthly instrument" -Heinrich Glarean
The trumpet marine isn't very trumpetlike in appearance. It's a long instrument with only one string. It's not even blown like a trumpet, yet it sounds kind like a trumpet. This neat trick is accomplished by playing the harmonics of the string, similar to the way a bugle or natural horn plays notes from the harmonic series. It also has a loosely fitted, buzzing bridge. This gives the trumpet marine an unexpected, brassy sound.
See Tromba Marina.com for additional information and sound samples.