A Viking ship has set sail for Dublin from the Danish port of Roskilde, in an attempt to recreate the voyages undertaken by early Norsemen.
The 30m (100ft) long replica, called Sea Stallion, is said to be the world's largest reconstructed Viking vessel.
It is based on a ship made nearly 1,000 years ago in Ireland, which in 1962 was excavated from the Roskilde fjord.
A crew of 65 volunteers will travel the 1,900km (1,200 miles) to Dublin, using only oar and sail power.
"The Vikings are coming back. Be prepared," Captain Carsten Hvid shouted before jumping onboard the ship.
The original, which was made in 1042, is believed to have taken part in clashes between the Anglo-Saxons and Normans in 1050-1060, when many Danish Vikings lived in Ireland.
The boat sank in the Roskilde fjord at the end of the 11th Century, while defending the country's coast from Norwegian Vikings.
The replica was constructed with around 300 oak trees and 7,000 iron nails and rivets.
It has a top speed 15-20 knots (about 30km/h or 20mph), but is equipped with GPS satellite navigation and radar. A modern support boat with medical and rescue experts will also accompany the ship.
The aim of the voyage is to sail uninterrupted to Dublin across the North Sea and around the north tip of Scotland.
The crew say the toughest challenge will be the weather, with no shelter on the open vessel from crashing waves, wind or rain.
"After all, we are testing the construction. Can it resist the rough weather? And the nights get really cold," Louise Henriksen, a 27-year-old historian and crew member, said.
"If we get 10 hours of rain and storms, we still need to be able to laugh to survive," said Claus Oreskov, 46, a Danish crew member.
"What all crew members have in common is that we all are a bit crazy."
The ship hopes to reach Dublin on 14 August.
The ship's crew will be writing a weekly diary for the BBC News website. More regular updates and a satellite map of the ship's latest position can be found at BBC History's Viking Voyage website.
The ship's voyage is also being filmed for a BBC Two Timewatch programme in the autumn.
THE SEA STALLION FROM GLENDALOUGH
1. The crew of 65 men and women will sleep on the open deck, as the Vikings did, and take turn keeping watch
2. Satellite navigation equipment will make sure the ship stays on course. Vikings had to rely on the position of the sun and stars, the colour and movement of the sea and wind direction
3. Oak planks were cut radially for maximum strength, overlapped and nailed together. Axes and other tools used to make the planks were replicas of those used by the Vikings
4. The sail, mast, rigging and rudder on the original were missing so these have been copied from other finds
5. Shields, vital in battle, were tied over the oarports when the ship was in port
Sources: Viking Ship Museum, Denmark; National Maritime Museum, UK. Photos: Werner Karrasch and Erwan Crouan