The link between seas is still a marvel, writes Bronwyn Elsmore.

Hands up those who think the Atlantic Ocean is higher than the Pacific Ocean. A few arms were raised.

"Hands up if you think the Pacific is higher than the Atlantic." More went up this time.

"Who thinks sea-level is sea-level no matter where it is."

It was a relief to see more votes cast for this one, but many still seemed unsure.

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The good thing was that 2000 passengers aboard the Dawn Princess could shortly extend their knowledge at first hand by seeing the Atlantic and Pacific Oceans in a single day courtesy of the Panama Canal.

All, including 266 Kiwis, took the opportunity early next day when we approached the Gatun Locks from the Caribbean side.

Every viewing space on the open decks was full, and the 400 balconies of upper deck cabins. But not all confusion had cleared, as some still clung to their misconceptions.

"Are we going up or down this way?"

And, despite the canal zone being free from the diseases that plagued its construction, "we started our malaria medication five days ago".

Thunder cracked above the hills at the northern end of the transit. Such storms produce 330cm of rain annually, providing the two billion gallons of fresh water needed by the canal each day, and securing the future of the Panamanian rainforest.

At 261m long and 32m wide, Dawn Princess was a Panamax ship — the maximum able to transit the three lock systems, then pass under The Bridge of the Americas at the Pacific end.

Only centimetres of water showed on either side as the locks were negotiated, not always precisely, so some repainting of the hull was necessary at subsequent ports.

Or perhaps the passengers, aware they had been misled on other matters, hadn't heeded warnings that the success of our passage depended on how much they ate on preceding days.

Many had prepared placards to be displayed for the planned video, or viewed by those at home watching the canal webcam.

Cards hung from railings, and the ship took on a party atmosphere.

"Hi to the Williams family". "Kia Ora whanau". "Debbie does Panama". "Cruising is a tough life, but we're prepared to make the sacrifice". "Kids, we're not coming home". "$end more money".

Some might have needed to display that last instruction as the cost of the canal transit was borne by passengers' fares.

Calculated at US$120 ($152) a berth, a preferential fee of $35,000, plus other charges, they totalled about $315,000. All the more reason to enjoy it.

Despite the heat, the party mood lasted. At the locks flags waved and greetings were shouted between ship and shore, or to the crew of a cargo ship in the canal. When, at Pedro Miguel locks, a balcony passenger threw chocolates from the nightly pillow treat to workers below, others followed so silver, red and blue-wrapped discs showered down.

The passage between the locks, gliding through Lakes Gatun and Miraflores, was time to spot crocodiles near the shore, and view turkey vultures circling above — birds so large and numerous that "Zonians" call them the "Panamanian Air Force".

It was also time to remember the human cost. During the first construction attempt by the French, more than 20,000 workers died. The passage near the site of the French cemetery brought a temporary sober tone to an otherwise bright day. Had labourer Paul Gauguin been among the casualties the world would have lost a further spot of colour.

By the time of the American project — pushed along by the gold rush, and military purposes during the Spanish-American war — technology and health matters had advanced. Fewer than 6000 further lives were lost by the opening date in 1914.

Still, the human cost of constructing this passage through the Americas is calculated at 500 lives for each mile of the canal.

With the centenary close, the scheme still works much as it did at its inauguration.

The "mules" — locomotives that guide the ships on either side — have improved over several generations. But after other methods were tried and failed, a rowboat still meets each ship at the entrance to the locks to transfer the guiding cables from shore to ship.

All too soon Dawn Princess passed into the Pacific, one of 37 ships to transit that day.

Even if the Panama Treaty didn't guarantee neutrality of the canal in perpetuity, its value as an earner would make it the country's highest priority. A new canal system is now being constructed in a $6 billion expansion to be completed in 2014, 100 years after the opening of the present system.

The Panama isthmus was formed naturally around three million years ago, becoming the bridge of life between the two Americas.

Thanks to the canal through the land bridge our ship made the transit between the continents during daylight hours.

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Further information: Most passenger companies offer cruises through the Panama Canal. For information about the canal, and a view of the canal webcams, see pancanal.com.