As an underwater archeologist living in the Great Lakes region, I seem to field the same question whenever I'm visiting the tropics: "What is there to see underwater up there?"

The answer is, of course, an easy one: Shipwrecks. Lots of beautifully preserved shipwrecks. This reply usually sparks plenty of interest and, more times than not, leads to a discussion of sunken ships. In recent years, the fascination with shipwrecks has grown considerably, due in part to blockbuster movies such as Titanic and popular documentaries about new underwater discoveries.

Beautiful and Dangerous . . .

The romance and adventure of shipwrecks have lured divers for generations. While many individuals indulge in exhaustive research about the maritime past, some divers also choose to view the past firsthand. As exciting as shipwreck diving is with treasures of history, adventure and sheer beauty, it's important to remember that potential hazards specific to these lures do exist.

Shipwreck diving can require several levels of experience and training. Depth of wrecks, currents, water temperature and visibility around wrecks can present varying degrees of hazards, and the kind of dive planned can determine the degree of risk involved.

Inspecting the open hull of a ship in 25 feet / 8 meters of clear, calm water and penetrating a Lake Superior schooner in 100 feet / 31 meters of frigid lake water are both shipwreck dives. Their levels of potential hazards and required training to dive them safely, however, vary greatly.

Hazards

Many people are content to visit shipwrecks without entering them. Staying outside the wreck makes for a much safer dive, but it hardly eliminates all potential hazards. Many things can affect divers who stay outside a wreck.

Entanglement

Shipwrecks, especially in waters with little or no vegetation, provide excellent habitats for fish. As a result, they become popular sites for fishing. Shallow wrecks tend to be littered with discarded lures, hooks and lots of fishing line, all of which can spell trouble for divers. In colder fresh water where preservation is ideal, a ship's rigging may remain intact, posing further entanglement perils. While other hazards may decrease in the shallower wrecks, entanglements from fishing lines, in particular, actually tend to increase. Consequently, dive with at least two sharp knives or cutting tools, even on very shallow wrecks.

And having the proper knives is only a first step, advises Bill Clendenen, Vice President of DAN Training. They must also be placed properly. Will you be able to reach your knife or cutting tool if you're entangled? You need to place your knife in an area where you can reach it without difficulty, Clendenen suggests. You may even want to consider two knives in different locations so they're easy to reach. Many wreck divers attach their knives on their BCDs, gauge consoles, arms or the inside of the leg.

Additionally, he notes, you should carry a tool that will cut through some of today's high-tech fishing nets and line. Your best bet against this type of netting is to carry a good pair of EMT shears that will cut through most lines you're likely to encounter.

Aquatic Animal Injuries

Serious injuries that result from accidental contact with marine life are rare on shipwrecks, but divers should realize the protection offered by a shipwreck creates an ideal habitat for many aquatic animals. In warm salt water, eels, lobsters and other creatures will make their home in wreckage. Divers can reduce their chances of injury by keeping their hands out of small openings where these creatures may live. Wear adequate exposure suits and gloves to help protect yourself against accidental contact with stinging, biting or piercing marine life.

Sharp Objects

Perhaps the most common hazard, injury from sharp objects can occur on any wreck. Exposed metal such as in hull spikes, engine components, rough metal edges, as well as wood fragments and coral growth, all can cause scrapes, cuts or punctures.

Even if the water temperature doesn't require it, you should have a good exposure suit with a hood and gloves when you're diving a wreck. Currents and surges are frequent hazards around wrecks, so you may have to brace yourself against being pushed into the wreck. Although a good exposure suit and gloves can minimize the hazard of sharp objects, good buoyancy control and establishing contact with your hands are the best ways to avoid injury. Drysuits can be punctured by over-weighted divers descending onto a nail or hull spike. To avoid injury when visiting wrecks in warm environments without exposure protection, stay out of the wreck altogether. Divers who visit wrecks frequently should also have a current tetanus immunization.

Strong Currents and Surge

Even on the most popular wrecks, strong currents and surge can unexpectedly overcome a diver. Getting pushed into the side of the wreck by a strong current or being raked across the top in strong surge can lead to injury.

Surge can be a real problem on shallower wrecks. Individuals who dive in strong surge should distance themselves from the wreck so they will not be pushed against it. In a wreck shallower than 10 feet / 3 meters and where the surge is strong, divers should establish positive buoyancy and view the wreck from the surface.

When you're diving on wrecks, you can use a variety of lines to help you make your dive safer. An ascent / descent line provides a valuable reference when making your safety stop - between 10 and 20 feet / 3 and 6 meters for three to five minutes - and it helps you make a controlled slow ascent. It also provides an excellent reference when navigating the wreck.

You can also hang extra air cylinders on an ascent / descent line in the rare event of an out-of-air situation. In limited visibility the descent line can guide you directly to the wreck, where you can evaluate the bottom current. If time and depth limits permit, you can choose to dive near the bottom of a wreck where currents tend to be weaker. Avoid doorways and hatches in strong currents: they can create a sucking action from the movement of water inside the wreck.

Penetration Hazards

The decision to enter a shipwreck should not be taken lightly by any diver. You can accomplish penetration many different levels, from simply swimming in and out of a large cargo hold, to navigating through doorways, stairwells and cabins.

You should make your first dive to a new wreck an inspection dive - don't try to penetrate the wreck on your first foray. By delaying penetration until a second or even third visit to the site, divers can adequately develop a safe dive plan based on elements of the site they observed previously. Wreck diving subjects the diver to many of the same risks as those found in cave or under ice diving. Therefore, entering any overhead environment requires specific planning, special equipment and advanced training; shipwrecks are no exception to this rule.

When a diver enters a wreck, the potential for hazards increases. If you intend to enter the wreck, the decision to enter it should be made well before the dive. Such an action involves specific planning, special equipment and advanced training.

When you enter a wreck, teamwork is an important part of your safety plan: As Buddy 1, you can lead the way into the wreck, using the wreck reel; your partner, Buddy 2, follows you, keeping the line taut. When it's time to leave the wreck, Buddy 2 leads the way back out.

If you plan to penetrate the wreck, you should carry at least two lights, notes Clendenen. Some divers carry three lights into the wreck, in keeping with cave diving rules and self-reliance guidelines. As a minimum, carry a reliable primary dive light and at least one backup if you're venturing inside. Be sure your lights are secure - you want to keep them from snagging on pieces of the wreck.

And remember, once you're inside a wreck, a diver can no longer make a direct ascent; this can greatly complicate a dive. Divers without the proper equipment and proper training in overhead environments should not enter a wreck.

Disorientation

For the unprepared diver, becoming disoriented in a wreck can be a serious problem. A lack of visibility from a silt-out (discussed below), or even forgetting the location of the entry point can leave a diver confused, disoriented and anxious.

Furthermore, in many cases, especially in wrecks made of steel or those with large amounts of iron connective hardware, dive compasses do not work properly. Thus, a wreck reel is mandatory for someone entering a wreck. A wreck reel has other uses such as mapping, navigating and, with a lift bag, it becomes a possible emergency ascent line.

Properly attaching the line at the entrance and regularly securing it to keep the line taut while you navigate the wreck is the best way to relocate the exit, even in low visibility. Wreck reels may be a hazard, however, if you allow the line to play out at random. Make sure to keep tension on the line when laying out and reeling it in to avoid entanglement.

Additionally, wreck reels need to fit the environment you're diving in - if you're diving in salt water, yours should be made of aluminum or stainless steel. As always, specialty diving demands special equipment and proper training.

Low Visibility and Silt-Outs

Reduced ambient light and silt present problems of visibility for the diver entering a shipwreck. Loss of ambient light occurs in any overhead environment, and wrecks are no exception. Remember to carry a primary flashlight and at least one backup light.

Silt can be a bit more complicated. Layers of silt or rust can coat the wreck floors, walls and ceilings. Silt-out, or mobile silt, caused by excessive movement of hands or fins, can result in reduced or limited visibility.

Low visibility inside a wreck can cause disorientation. Risks of entanglements and injury from sharp objects increase in a low- or no-visibility environment: the diver simply may not be able to detect them.

Silt-out can cause anxiety for inexperienced divers or for any divers not properly trained to dive in these environments. Anxiety can become dangerous in a confined, overhead environment, especially if it leads to panic.

With proper techniques, you may be able to avoid silt-outs altogether. Divers with good buoyancy control can avoid bumping into the floor or ceiling; you can also develop a kick that minimizes fin movement, displacing less water and therefore dislodging smaller amounts of silt. If a line or railing is present, divers can use either to pull themselves through without kicking at all. Over time, dislodged silt and rust will generally settle out.

Keep in mind that you need to dive with a buddy whose training and skills are similar to that of your own. You may practice excellent buoyancy and finning skills, but the wrong partner can still cause an anxious moment - or worse.

If silt becomes too much of a problem during a dive, a diver can stop all movement and wait for improved visibility. If visibility does not improve, divers can follow their reel lines back to an area of better visibility or to the exit. Often, a flashlight's beam will reflect off the particulate and further reduce visibility. If the diver turns off the flashlight, visibility can actually improve.

Internal Water Movement

A diver might think that once inside a wreck, he will be protected from currents and surge, but that's not always so. In fact, currents or surge entering a ship will often increase speed and strength as they are forced through narrow openings. Movement of water inside the wreck can cause several problems: it might slam a diver against a hard or sharp object; it might force the diver against a wall; and it might cause a silt-out. To avoid such hazards, divers should simply evaluate the current outside the wreck and, if the current is too strong, avoid entering the wreck.

Equipment Snags

When someone navigates through a wreck, everything from weight belts to regulator hoses can get snagged. Divers should address this hazard before the dive and secure all gauges, hoses and weight belts. You should also have an available, easily accessible alternate air source, should a regulator hose become snagged or the regulator ripped from your mouth.

Clendenen notes that snaps and other clips used to attach extra equipment necessary for safe wreck diving can also carry some risk. Be sure to use non-corrosive snaps with a sliding gate (bolt snap) and not a quick-release snap (snap hook). Snaps with a quick-release mechanism can catch on wreck lines, entangling the diver, or they may dislodge a critical piece of wreck diving equipment like a light or reel. If you must use a quick-release snap, make sure you face the clipping mechanism towards you when you dive.

If you choose to penetrate a wreck, you may want to dive with two weight-belt buckles, an extra precaution because your weight belt buckle can snag on a part of the wreck and be dislodged. Choose a weight-belt buckle made of metal instead of plastic, because plastic buckles are more prone to cracking and breaking.

Remember to monitor your breathing gas supply when you're wreck diving; and consider having a separate gas supply in the unlikely event your primary system fails. Whether you choose to dive with a pony bottle or doubles, it's important to ask: "Will I have enough gas to get me to the surface, including stops?" Which backup system you choose is largely dependent on the wreck itself and the individual risks that are posed. Make sure your breathing system is well-maintained and serviced regularly.

You may consider using a DIN fitting on your regulators and cylinder valves; this helps prevent accidental dislodging if you bump the top of your tank on part of the wreck. The captured O-ring in the regulator reduces the risks of O-ring extrusion and failure while inside or around the wreck.

A Few Extra Tips

Most experienced divers agree that many problems can be solved long before the dive begins. Proper training, teamwork, good equipment, thoughtful planning and smart decisions reduce the risk. Discuss your decision to enter a wreck at the pre-dive planning stage, and avoid changing your plans during the dive. Don't enter a wreck if you haven't made plans to do so ahead of time; be properly equipped and mentally prepared to make the dive.

Before a dive, experienced divers discuss potential hazards and devise ways to alert others of any observed hazards. Many wreck divers regularly use specific hand signals for hazards. As in any dive, you should obey strict time and depth limits and follow proper ascent rates.

Shipwrecks are time capsules, and, to explore them is to visit a museum. We must make every effort to visit them safely - for their sake and for our own.

- With Reports from Bill Clendenen, Vice President, DAN Training