According to GAT (PRN 4699) Carn Arthur, a possible Crannog, can be found a little beyond the (current) south-western end of Llyn Mwyngil at SH71000900 - that is where the Afon Dysynni makes a loop near a weir:
'Stony mound.....referred to in a reliable C19th account as being visible at low water and a good place to fish'. Sources: Peterson, R. & Roberts, J. G. , 1989 , Archaeology in Wales.
Another site added that gives me an excuse to extol the virtues of Susan Cooper's "Dark Is Rising" sequence. The fourth part, The Grey King, features Llyn Mwyngil at its climax, where six knights (the "sleepers") rise from the waters of the lake to overcome the power of the Brenin Llwyd, the Grey King of Cadair Idris.
The lake is also referred to in one of the poems that feature through the sequence: "By the pleasant lake the sleepers lie".
There have been - to my knowledge - three major discoveries of Iron Age hoards in Wales to date: namely within the Llyn Fawr above The Rhondda... at Llyn Cerrig Bach, Anglesey..... and here-abouts.
The Llyn Fawr is strikingly evocative and Anglesey is, well, Anglesey; however the southern flank of Cadair Idris rising above Llyn Mwyngil (better known - unfortunately for those with due respect for the vernacular grammar - as Tal-y-Llyn Lake) is, in my opinion, a truly classic location of stunning aesthetic appeal, the craggy mountainside sweeping elegantly - albeit with a maverick 'roughness' anticipating the heights of central Snowdonia further north - down to lakeside from near enough 3,000ft. It's a disappointment to note that the metal alloy artefacts deposited here were not actually recovered from the depths of the lake - as in Authurian lore - but from the nearby crags in 1963 (I understand in the vicinity of the Nant Cadair, outflow of the utterly wondrous Llyn Cau), hidden beneath a boulder... hence casting significant doubt as to whether the hoard represented a votive offering to the Celtic gods, or ill gotten gains stashed.... and never reclaimed. Hey, perhaps retrieved from the llyn by an unscrupulous rogue in search of a quick profit? (or, an intriguing thought.... was the primary origin actually Llyn Cau?) Who knows. Who will ever know? I reckon the latter, myself, bearing in mind the piece of 'lock'. Still, the treasures now reside in The National Museum of Wales, Cardiff. So go see 'em, if you get the chance. But remember where they came from. Oh yes. The landscape that inspired the vision. And how...
Of primary interest is a trapezoid brass 'repousse' plaque, (according to the experts at the museum) apparently designed as one of a pair, representing a human face with 'staring eyes, and finely combed hair, an image of striking quality'. The hoard also contained 'fragments from two shields', including a boss, some plates, perhaps originating from a 'ceremonial cart'... and 'part of a Roman lock'. However, the quality of workmanship notwithstanding, it is where the metalwork came from that, for me, engenders such interest, the salient detail that ensures such vitality of design and form 'means what it says on the tin' - or brass - that this was for real. If you should happen to find yourself standing at water's edge here... take a long, lingering look at the rocky heights to left and right... note the chasm in the cliff line above Minffordd where flows the Nant Cadair.... and the high bwlch of Llyn Bach carrying the road toward Dolgellau to the north-east... and see what you think? Better still, put on the boots and have a wander up to Cwm Cau.