In a sense, any dive site that boasts a shipwreck is an underwater museum. Frozen in time, a wreck is a snapshot of maritime technology. However, more than just a glimpse at a ship's engineering, underwater archaeologists are able to learn about the culture that used it; reconstruct ancient shipping routes; or unravel some of history's mysteries. Even in the best case scenario, though, most sites only have one or two wrecks, which most people wouldn't call a "museum."

However, there are a number of places where harbors have flooded, or entire cities have dissolved into the sea, allowing history buffs, scuba divers, or average tourists to travel into the past and see what life was like "back then." The real beauty of underwater museums, though, is that while "regular" museums remove ancient artifacts from their resting sites, submerged museums preserve them, intact, and in context.

1. The most recent entrant into the world of underwater museums is in the Mediterranean. In Israel's coastal city of Caesarea -- the site of "Herod's Harbor" -- visitors can inspect what was once one of the biggest and most sophisticated ports of the Roman Empire. Originally opened for business in 10 BC, the submerged harbor now rests in about 20 feet of water, and the museum itself covers some 18,580 square feet. With waterproof maps and an instructor to guide them, divers and snorkelers can swim among the 36 exhibits -- from sunken vessels, to giant anchors, to marble columns -- by following ropes tied between poles stuck in the sea floor.
2. Located off Alexandria, Egypt, the Underwater City of Cleopatra dates to 300 BC. Resting both inside and outside the harbor, the city crumbled into the sea after several earthquakes. Sitting between 15 and 45 feet of water, adventure-seekers can see ancient wrecks, sunken sphinxes, broken columns, a hieroglyph-covered obelisk, and thousands of granite blocks around Pharos Island (check out this great video of Pharos). If you're interested, Divernet has an excellent summary of the sites, as well as some of the potential hurdles they must overcome.
3. The Florida Keys National Marine Sanctuary, boasts the Shipwreck Trail, which features 9 shipwrecks spread across several miles, so you're not going to see all the wrecks at one go. Diving conditions vary from easy dives in shallow water to deeper dives of l00 feet or more. For each of the 9 wrecks, divers can access an underwater guide indicating the mooring positions; a history of the ship; a site map; and potential marine life. For people in the UK, the Hampshire and Wight Trust for Maritime Archaeology has set up a similar, 20-foot-deep archaeological trail off the Isle of Wight, which allows divers to move between numbered stations on the sunken HMS Pomone, a 38-gun frigate. 
4. The Spanish galleons Guadalupe and Tolosa sank in the Dominican Republic's Samana Bay during a hurricane in 1724. Undiscovered until the 1970s, the wrecks lay undisturbed for centuries; however, in the 1970s, they were removed for safekeeping. Recently, though, Indiana University students and faculty carefully returned the ballast stones, cannons, cannonballs, ceramic pieces and one anchor to the site. Currently resting in 12 to 15 feet of water, the quarter-acre site is easily accessible to snorkelers and divers.
5. The main portion of China's new Baiheliang Underwater Museum was completed recently. When fully constructed, the museum will protect the 18 fish figurines and more than 30,000 characters of poems carved over the past 1,200 years to measure the water level in the Yangtze River during low water seasons.