Isles of Scilly, United Kingdom

It is likely that, until relatively recent times, the islands were much larger, and perhaps conjoined into one island named Ennor. Rising sea levels flooded the central plain around 400–500 AD, forming the current 55 islands and islets (if an island is defined as "land surrounded by water at high tide and supporting land vegetation"). The word Ennor is a contraction of the Old Cornish En Noer (Doer, mutated to Noer), meaning 'the land' or 'the great island'.

A description, written during Ancient Roman times, designates Scilly "Scillonia insula" in the singular, indicating either a single island or an island much bigger than any of the others.

Remains of a prehistoric farm have been found on Nornour (now a small, rocky skerry far too small for farming). There once was an Iron Age British community here that continued into Roman times. This community was likely formed by immigrants from Brittany—probably the Veneti—who were active in the tin trade that originated in mining activity in Cornwall and Devon.

At certain low tides, the sea becomes shallow enough for people to walk between some of the islands. This is possibly one of the sources for stories of "drowned lands", e.g. Lyonesse.

Ancient field walls are visible below the high tideline off some of the islands, such as Samson.

Some of the Cornish-language place names also appear to reflect historical shorelines and former land areas.

The whole of southern England has been steadily sinking, in opposition to post-glacial rebound in Scotland: this has caused the rias (drowned river valleys) on the southern Cornish coast, e.g. River Fal and the Tamar Estuary.

Offshore, midway between Land's End and the Isles of Scilly, is the supposed location of the mythical lost land of Lyonesse, referred to in Arthurian literature (of which Tristan is said to have been a prince). This may be a folk memory of inundated lands, but this legend is also common among the Brythonic peoples; the legend of Ys is a parallel and cognate legend in Brittany, as is that of Cantre'r Gwaelod in Wales.

In 995, Olaf Tryggvason became King Olaf I of Norway. Born c. 960, Olaf had raided various European cities and fought in several wars. In 986 he met a Christian seer on the Isles of Scilly. He was probably a follower of Priscillian and part of the tiny Christian community that was exiled here from Spain by Emperor Maximus for Priscillianism. In Snorri Sturluson's Royal Sagas of Norway, it is stated that this seer told him:

Thou wilt become a renowned king, and do celebrated deeds. Many men wilt thou bring to faith and baptism, and both to thy own and others' good; and that thou mayst have no doubt of the truth of this answer, listen to these tokens. When thou comest to thy ships many of thy people will conspire against thee, and then a battle will follow in which many of thy men will fall, and thou wilt be wounded almost to death, and carried upon a shield to thy ship; yet after seven days thou shalt be well of thy wounds, and immediately thou shalt let thyself be baptised.

The legend continues that, as the seer foretold, Olaf was attacked by a group of mutineers upon returning to his ships. As soon as he had recovered from his wounds, he let himself be baptised. He then stopped raiding Christian cities, and lived in England and Ireland. In 995, he used an opportunity to return to Norway. When he arrived, the Haakon Jarl was facing a revolt. Olaf Tryggvason persuaded the rebels to accept him as their king, and Jarl Haakon was murdered by his own slave, while he was hiding from the rebels in a pig sty.

With the Norman Conquest, the Isles of Scilly came more under centralised Norman control. About 20 years later, the Domesday survey was conducted. The islands would have formed part of the "Exeter Domesday" circuit, which included Cornwall, Devon, Dorset, Somerset, and Wiltshire.

In the mid-12th century, there was reportedly a Viking attack on the Isles of Scilly, called Syllingar by the Norse, recorded in the Orkneyinga saga—Sweyn Asleifsson "went south, under Ireland, and seized a barge belonging to some monks in Syllingar and plundered it." (Chap LXXIII)

... the three chiefs—Swein, Þorbjörn and Eirik—went out on a plundering expedition. They went first to the Suðreyar [Hebrides], and all along the west to the Syllingar, where they gained a great victory in Maríuhöfn on Columba's-mass [9 June], and took much booty. Then they returned to the Orkneys.

"Maríuhöfn" literally means "Mary's Harbour/Haven". The name does not make it clear if it referred to a harbour on a larger island than today's St Mary's, or a whole island.

It is generally considered that Cornwall, and possibly the Isles of Scilly, came under the dominion of the English Crown for a period until the Norman conquest, late in the reign of Æthelstan (r. 924–939). In early times one group of islands was in the possession of a confederacy of hermits. King Henry I (r. 1100–1135) gave it to the abbey of Tavistock who established a priory on Tresco, which was abolished at the Reformation.