"Maybe we'll find real treasure here," said Jerra-Mary. "Just look at all those castles", remarked Keswick. "They must have been built to guard all the gold!"

A look around the ruins soon dispelled that theory - but the local children told them to seek out Old Harry - he knows everything about Portobello. Old Harry was easy to find and, sure enough, he told them all about the wonders of Portobello and the Spanish Main - and the Four were even allowed to hold some real treasure, Spanish Doubloons.

The next day they collected their pilot Carlos, who helped them transit the canal and also told them all about it. "Firstly you are going to enter the first of six locks. A lock allows huge ships to be lifted over hills and, in this case, across the finger of land that joins North and South America," he explained.

"Each lock is 305m long and 34m wide huge… Poco Andante at 13.5 m long and 4m wide will be lost inside it! They usually tie three yachts together as a raft and put three rafts in each lock - making 9 in all," continued Carlos.

The Four, as always, helped out where they could - this time they were to act as line handlers. Each raft is held in position by 4 long ropes tied to the side of the lock.

The entrance into the lock was awe inspiring with lots of activity everywhere.

"What are those men carrying?" asked Keswick. Before Carlos had time to answer a hard ball came flying over and landed with a clunk on the deck. "Grab that!" exclaimed Carlos, "and tie it onto the end of your line and then let it go."

"That's our line man - he will take our line and loop it around the bollard." With that done the Four could admire the view. The huge gates closed and the surrounding walls formed a huge box that slowly filled with water.

In the lock alongside a giant cargo ship was being pulled into the lock by little silver engines.

"They're called mules," informed Carlos. "Aren't they cute," said Little Bear, who was mesmerised by the dance of these little engines.

"Pull the line in!" shouted Carlos. "We're drifting too close to the far wall." The Four pulled with all their might and slowly pulled Poco Andante and her raft back to the centre of the lock.

After the first lock things became easier. The lines men held onto the lines and walked them along to the next chamber as the boats motored slowly forward. The gate closed behind them and the next huge lock filled with water. After the third one finally opened the boats had been lifted nearly 100m to a huge lake that stretched for miles.

Poco Andante motored on and the four could enjoy the sites; parrots, howler monkeys calling from the branches, and alligators swimming in the water - all unafraid of the huge ships passing them by.

"It's a special machine for making the canal bigger," replied Carlos. "They're making the canal wider to take even bigger ships. Look, you can see the trucks taking all the soil away and there is even a special barge that can put dynamite into the rocks."

The Four continued on under bridges and passing all manner of ships and barges until they came to the second group of locks, this time going down to the Pacific.

"This is part is easier," remarked Jerra-Mary, after they had passed into the second of the three Miraflores Locks.

Soon this adventure was over and the Four passed under the Bridge of the Americas and entered the Pacific Ocean. "A new adventure starts here," said Keswick as the Four gazed west to this vast ocean.