Das Altes Schiff Museum is a kind of time machine. During the construction of an annex to the Mainz Hilton Hotel, the remains of seven ancient roman ships were discovered entombed in the muddy banks of the Rhine. These have been carefully preserved and analysed.
Two of the vessels were warships and these have been reconstructed at full size. The others were cargo barges and scale models show what they were originally like.
There are many interactive exhibits and a couple of interesting films. It is well worth a visit. You can get there by SBahn.(Romisches Teatre station) or bus or just walk north along the Rhine bank.