I'm still wondering about some unreachable places of Summerset (after the "Potansa hypothesis" before the Grand Psijic Villa... X) ) and there's that island off the northeastern coast of Summerset mainland.
You may have noticed it, it's not accessible nor have a name:





In fact it may be called "Belport Run" (or some variation of this).

The more you know...
This is often useful when you're roleplaying and looking for "blank" places for you own background. (Then the risk now is dozens of RPers making their separate fanon about Belport haha )

https://elderscrolls.wikia.com/wiki/Belport_Run_(Arena)?file=Belport_Run.png

Here's a mere fan insight about worldbuilding...
When I wrote on the environments in Summerset on the General forum I started to compare the ESO:Summerset locations with the former lore of TES:Arena locations. (I'll put brackets for Auridon places.)

- Some invariant names:
Alinor
Corgrad Wastes
Crystal Tower
Cloudrest
Ebon Stadmont
Lillandril
Sea Keep
Shimmerene
Sunhold
[Firsthold]
[Skywatch]
[Vulkhel Guard]

- Some obvious ones:
Dusk <= Dusk Keep
Karnwasten Moor <= Karnwasten
King's Haven <= King's Haven Pass
Wasten Coridale <= Wasten Coridale or Wasten Coraldale (*)
[Silsailen Point] <= [Silsailen]

- Quite obvious too, but "reverse-engineered" to look like in-universe misspelling:
Silver Wood <= Sil-Var-Woad (then no silver forest there... but a zoo)
Thorheim Guard (quite unlikely to appear because of real-life naming from Thor) <= Tor-Hame-Khard
Rosefield <= Russafeld (then no persian-like rose fields but vineyards ...there's still rose-flowered pastures)

- Interestingly different, while having a similar name :
Marbruk Brook (Summerset island in tESA) / Marbruk (altmer colony in Greenshade, Valenwood, in tESO)

- Some without an equivalent in ESO, but that you could identify to some places thanks to the map:
Archen Grangrove <= Colossal Aldmeri Grotto ? (if it means its "arches") or Archon's Grove ? (according to the wiki writer, it seems to match fairly well too)
Belport Run <= (yes, that island)
Glenview <= (likely part of the Sunhold public dungeon)
Graddun Spring <= (hidden part near Karnwasten)
Holly Falls <= waterfalls between Archon's Grove (kind of confirmed by the sacred flowers of Mara there) and Alaxon'ald
Karndar Watch <= (nothing apart from the Direnni geyser and acropolis or I missed sth?)
Marnor Keep <= (likely part of the Sunhold public dungeon)
Old Falls <= waterfalls at the entrance point of the Cloudrest raid
Riverfield <= Rellenthil
Riverwatch <= Cey-Tarn
[West Guard] <= [Greenwater Cove] or [Dawnbreak]
White Guard <= (may be part of Sea Keep ... or reverse-named and displaced as "Veyond Wyte", a wayshrine on north coast)


(*) Wasten Coridale is known for its Hanging Gardens, which is located in the center of the Coridale township. The gardens are riddled with Elden fountains and ancient chains, with representations of the Earth-Bones and their transient rules. - quoting the elder scrolls wiki
Indeed there are unreachable islands and buildings (aldmeri ruins at least) off the beach of the delve.