Seileag, the Loch Shiel creature: three humps seen from the steamer Clanranald opposite Gaskan

Loch Shiel is long, narrow and deep. It runs about 28 km south-west from Glenfinnan to Acharacle, and much of it can only be reached by boat. Its maximum depth is given as 128 m, or 120 m in Murray and Pullar's 1910 survey. Through the River Shiel, it drains to the sea.

The loch had a lifeline. The mail steamer Clanranald II, 70 ft long and 50 tons gross, ran daily from Acharacle to Glenfinnan, calling at Polloch and Dalilea and carrying mail to and from the West Highland Railway. She had replaced the first Clanranald, which drew too much water, and she served the loch from 1900 to 1953. On the north shore stood Gaskan. In the 1870s the Ordnance Survey recorded it as "two or three small houses." The MacDonalds are said to have built their boats there.

1905. The steamer was opposite Gaskan. Four people were travelling on board. Ewen Mackintosh, named first in every version of the story. Ian Crookback, remembered as an old man. And two young boys whose names were never recorded.

Through a telescope, all four saw the same thing. Three humps above the water. One retelling says the humps broke the surface. That is all the account preserves. No head. No neck. No colour. Nothing about how long the humps stayed in view, or where they went.

The report survives because of Father Henry Cyril Dieckhoff, a Benedictine monk at Fort Augustus Abbey. He gathered sightings from Loch Shiel, Loch Quoich and other lochs for a book he never finished. He has been called Seileag's "most diligent investigator." Dieckhoff died in 1970. His notes are now unaccounted for.

The 1905 sighting reached print only through later writers, among them Peter Costello in In Search of Lake Monsters and Michael Newton in his Encyclopedia of Cryptozoology. Karl Shuker gave the fullest modern version in 2015. No newspaper of the time is known to have reported it. No photograph exists.

No writer has offered an explanation for this sighting in particular. The standard explanations for humped shapes on a Scottish loch include standing waves from a boat's own wake, a real possibility for witnesses aboard a moving 70 ft steamer. Swimming otters or deer have been suggested for such reports, and so have seals, which could reach the loch from the sea by the River Shiel. So have lines of waterbirds, floating logs and wind slicks seen at a distance.

Against all of this stands the agreement of the witnesses. Four people, an old man among them, looked through a telescope. All four saw three humps.

Seileag takes her name from the Gaelic an t-Seileag, a feminine diminutive derived from the name of the loch. She is also called Sheila. In Hebridean tradition the seileag is a huge eel that lives in the sea and sometimes enters lochs.

The 1905 humps were not the last. In 1911 a groundskeeper saw a three-humped beast through a telescope. In 1926 Ronald MacLeod watched a huge creature with "seven sails" on its back rise from the water and come out at Sandy Point, bigger, he said, than the Clanranald. On 9 June 1998 staff and guests at the Glenfinnan House Hotel saw a disturbance in the water. Seileag stands in quiet company with Morag of Loch Morar and Nessie, one of the lesser monsters of the Highland lochs.