The Silfra fissure, is known as one of the top dive sites in the world two main reasons.

First, the Silfra fissure is actually a crack between the North American and Eurasian continents, meaning that you dive or snorkel right where the continental plates meet and drift apart about 2cm per year.

Silfra is the only place where one can dive or snorkel directly in the crack between two continental plates.

Secondly, the underwater visibility in the Silfra fissure is over 100 meters, which creates an underwater experience that will rarely, if ever, be surpassed. The reasons for this astounding water clarity are twofold: the water is cold (2°C – 4°C year round ) as it is glacial water from the nearby Langjökull and this water is filtered through porous underground lava for 30-100 years until it reaches the north end of Thingvellir lake, seeping out from underground wells. The Silfra water is as pristine as water can get and you can drink it at anytime during your dive or snorkel.

The Silfra fissure consists of four sections: Silfra Big Crack, Silfra Hall, Silfra Cathedral, and Silfra Lagoon.  We plan our dives and snorkel swims so that we are able to see all Silfra sections in every Diving Silfra Day Tour and our Silfra Snorkeling Tour.  We enter the water from a platform with steps leading down.  If you are diving, the maximum depth of the your dive in Silfra will be 18 meters, but the average depth of the dive is between 7 and 12 meters.

Although Thingvellir Lake has an abundance of fish species and trout fishing is very popular in the lake, the fish usually do not venture far into the Silfra fissure.  The marine life in Silfra consists mostly of bright green “troll hair” and different types of algae that provide a colorscape unlike anything that occurs naturally above the surface.

The National Park Thingvellir has been declared a UNESCO WORLD HERITAGE SITE both for its cultural and historical significance as well as natural and geological uniqueness.  It is well worth it to join our Golden Circle Day Tour to further explore Thingvellir on land.  Moreover, if you have friends or family accompanying you on your tour but do not wish to get in the water themselves, the area around Silfra is full of lovely walking trails that lead through this fascinating place.

"Nobody understands the allure of the sea more than the U.S. Coast Guard, but we also see the tragic results when people underestimate the hazards. The adventure and thrill of diving are appealing to many, but the ocean is an unforgiving environment — and even less forgiving to those who recreate beneath the surface."

— Rear Adm. Karl Schultz, commander of the 11th Coast Guard District

Recreational diving is by and large a safe activity, but when accidents occur the outcomes are often frightening and can be fatal. The beautiful blue world below can quickly become hostile for divers who lack adequate training, are in poor physical condition, use improperly maintained equipment or are otherwise unprepared.

Although the U.S. Coast Guard does not have regulatory authority over recreational diving as it does for recreational and commercial boating, Coast Guard search-and-rescue crews are frequently called on to assist when divers are lost or in trouble. In the aftermath of a dive injury or death, the Coast Guard marine casualty investigators work with other public health and safety organizations to identify what went wrong and evaluate how to prevent future accidents.

In 2009 the Coast Guard began to forge strong partnerships with the San Diego Lifeguard Services, the San Diego Harbor Police, the San Diego County Medical Examiner's Office, the University of California San Diego Health System and the Scripps Institution of Oceanography to analyze dive incidents. The committee formed by these groups produced six recommendations based on a comprehensive review of diver fatalities in the San Diego area. The committee encourages divers everywhere to ask themselves the following questions:

1. Is your training adequate for the current and predicted conditions? Will you respect the limitations created by the conditions and stop diving when conditions change or exceed your personal limits?All the normal hazards of water sports are magnified for those who spend time beneath the surface. Strong currents can occur at any time of year. Cold water temperatures, limited air supply, reliance on equipment for survival and the lack of underwater rescue capabilities make it essential that divers are fully aware of their limits and prepared for all possible problems.

2. Are you prepared to abandon your weights, inflate your buoyancy compensator and signal for help when in distress?Divers should not be afraid to ditch their weights, end their dives and signal for help at the first signs of distress. Interviews with divers who have experienced distress reveal that many of them did not understand they were in danger because they had not been taught how it would feel; therefore, divers should signal for help if they have any concerns at all.

3. Is your physical fitness adequate for the current and predicted conditions? Have you checked with your primary doctor to ensure that you are in good enough health for intense physical exertion? Diving is a strenuous physical activity involving physiological demands unlike those of any other sport. Many dive fatalities are caused by heart attacks, and the risk is especially great for divers over the age of 45. Divers who have not dived in more than a year should consult with their primary-care physicians before attempting to return to the sport. They should then reassess their abilities with a simple or less-challenging dive.

4. Are you diving with a buddy? Have you reviewed each other's abilities, equipment and plans?In addition to planning, health, physical fitness and awareness of weather and sea conditions, dive-safety experts stress the importance of the buddy system. Divers should never dive alone. They should always have detailed plans (which include times and locations) that they share with someone ashore.

5. Do you feel completely comfortable making this dive?It is essential to prioritize safety and remain realistic about upcoming dives. Any hesitations about any aspect of a dive should be completely resolved prior to commencing the dive. Divers should also clearly understand their experience levels and only attempt to exceed these limits when the conditions are optimal and they are diving with more experienced partners.

6. Do you plan to enter overhead environments? If so, do you have the proper training and equipment, and are you familiar with the necessary procedures?Diving in caves, wrecks or any overhead environments in which the path to the surface is indirect necessitates additional training, equipment and air supply. In overhead environments, prepare yourself for confined spaces, entanglement and disorientation.

According to the Coast Guard's Tactical and Strategic Statistics, in the past four years (2010-2013) the Coast Guard was called for assistance in 63 fatal dive accidents and 55 diving-related injuries. We hope that publishing these safety tips will lead to fewer dive-related tragedies. "The Coast Guard doesn't regulate recreational diving but is generally called in to assist during diving emergencies," Schultz said. "In many of these dive emergencies, injuries and death are preventable. We want everyone who enjoys the water, including divers (whose sport leaves little room for error), to make safety their top priority. We want you to survive your dive."

The problem is, they're not exactly sure what it is they've uncovered.

Out searching for shipwrecks at a secret location between Sweden and Finland, the deep-sea salvage company Ocean Explorer captured an incredible image more than 80 meters below the water's surface.

At first glance, team leader and commercial diver Peter Lindberg joked that his crew had just discovered an unidentified flying object, or UFO.

"I have been doing this for nearly 20 years so I have a seen a few objects on the bottom, but nothing like this," said Lindberg.

"We had been out for nine days and we were quite tired and we were on our way home, but we made a final run with a sonar fish and suddenly this thing turned up," he continued.

Using side-scan sonar, the team found a 60-meter diameter cylinder-shaped object, with a rigid tail 400 meters long.

The imaging technique involves pulling a sonar "towfish" -- that essentially looks sideways underwater - behind a boat, where it creates sound echoes to map the sea floor below.

On another pass over the object, the sonar showed a second disc-like shape 200 meters away.

See also: Quest for Sir Francis Drake's remains

Lindberg's team believe they are too big to have fallen off a ship or be part of a wreck, but it's anyone's guess what could be down there.

"We've heard lots of different kinds of explanations, from George Lucas's spaceship -- the Millennium Falcon -- to 'it's some kind of plug to the inner world,' like it should be hell down there or something.

"But we won't know until we have been down there," said Lindberg.

The Head of Archaeology at Sweden's Maritime Museums, Andreas Olsson, admits he's intrigued by the picture, but remains sceptical about what it could be.

The reliability of one-side scan sonar images is one of his main concerns, making it difficult to determine if the object is a natural geological formation or something different altogether.

"It all depends on the circumstances when you actually tow the [sonar] fish after the boat," he said.

"What are the temperature conditions, the wave conditions, how deep is your fish in relation to the sea bed etcetera and all those parameters also affects what kind of image you have in the end," he explained.

Even Lindberg agrees the image "isn't the best it could be." But his crew are still planning to return to the site in the calmer waters of spring to investigate their find.

It's a risky and expensive business, and not one that always pays off.

British maritime historian, Professor Andrew Lambert, says the costs of recovery are now too high for most.

"If you want to stand in a cold shower tearing up £50 notes, go shipwreck hunting," he said. "Most shipwrecks are rotting away, or carrying dull things -- all the romance has been taken out of it."

It's a problem Lindberg and his team are aware of.

"It's a very difficult industry to be in -- it's money all the time," he confessed. "The best thing it could be, would be 60 meters of gold -- then I would be very happy."

"This thing is very far out, it's really off-shore, so first of all we need a bigger ship... more equipment.. and we have to do bottom sampling, water sampling, to see if it is something poisonous."

But even if the mystery object doesn't contain retrievable treasure the site could still prove to be a gold mine for the Ocean Explorer team, with tourists and private investors paying to see it up-close, in a submarine.

"The object itself is maybe not valuable in the sense of money it can be very interesting whatever it is, historical or a natural anomaly," said Lindberg.

In the North Atlantic, one American salvage company is also hoping to beat the odds.

Using side-scan sonar, the team found a 60-meter diameter cylinder-shaped object, with a rigid tail 400 meters long.

Odyssey Marine Exploration -- a company made up of researchers, scientists, technicians and archaeologists -- have at least 6,300 shipwrecks in their database that they are looking to find.

Their latest discoveries include two British war-time shipwrecks off the coast of Ireland that could be laden with hundreds of tonnes of silver.

Mark Gordon, president of Odyssey, says at least 100 ships on their watch-list are known to have values in excess of $50 million dollars.

"When you think about the fact until the mid 20th century, the only way to transport wealth was on the oceans and a lot of ships were lost, it adds up to a formula where we have billions of dollars worth of interesting and valuable things on the sea floor," he said.

The lure of treasure has lead to an increasing number of discoveries in recent years. But one which doesn't come without its dangers, warns Olsson.

"I think recently we're entering a time of a lot of discoveries," he said of the technological advancements in finding shipwrecks.

"The professional shipwreck discoverers are doing a great effort for cultural heritage management in the long run... what we don't support is the action of actually taking up items and selling them," he said.

Conventional wisdom has long held that corals -- whose calcium-carbonate skeletons form the foundation of coral reefs -- are passive organisms that rely entirely on ocean currents to deliver dissolved substances, such as nutrients and oxygen. But now scientists at MIT and the Weizmann Institute of Science (WIS) in Israel have found that they are far from passive, engineering their environment to sweep water into turbulent patterns that greatly enhance their ability to exchange nutrients and dissolved gases with their environment.