Thanks Martin! So Ceuta and Spain could very well be linked by
Spanish territorial sea.

You mentioning the fact that the states adjoining the Strait of
Gibraltar may not under any circumstances impede the free passage of
ships brought me to another (maybe only slightly similar) issue: on
dry land, sometimes a road or railroad, 'belonging' to one country (I
mean it is exclusively used by that country, and probably belongs to
it as well, although it could very well not be the case) passes
through another country. Some examples (probably not all of the same
category):

The Dutch road N274 from Koningsbosch to Brunssum, passing through
Germany but never actually level-crossing a German road (no junctions
either). With other words: you can't get off this road while in
Germany. This is a legacy from the time the Selfkant was Dutch
(although at that time, there probably were normal road crossings).

The Swiss road from Basle to the Swiss part of Basle-Mulhouse airport
(I remember seeing this on a map once. You can't get off in France,
but you won't have to pass a French border post either to get to the
airport).

The German railroad between Neusalza-Spremberg and Taubenheim,
running for a short stretch through the Czech rep. (the so-
called "Schluckenauer Zipfel").

The German railroad between Zittau and Goerlitz runs through Poland
for two stretches. One of them is linked to the main net in Poland,
but there is another, very short stretch (near Trzciniec),
shortcutting a bend in the river Neisse, too.

The railroad from Adorf (D) to Cheb (CZ) crosses the border five
times, instead of just once! From the German border station of Bad
Brambach to the north, one has to pass a stretch in Czech territory
first. From the Czech border station of Plesna (acute on a) to the
south, you pass through a piece of Germany before entering the Czech
rep. again.

I think the road from Monschau to Aachen is another one. It runs
through a piece of Belgium near Simmerath. But then again, there are
some level crossings in this road with the Vennbahn, so the situation
is much more complicated here.

The border agreement between Croatia and Bosnia springs to mind.
Bosnia lets Croatian travel freely from one part of Croatia to
another, passing the tiny Bosnian coastal area. In return, Bosnia can
use, without Croation border control getting in the way, the nearby
Croatian harbour of Ploce (caron on c).

I'm not sure whether all this falls within the scope of this
discussion group, but since the question of access/free passage comes
up now and again, I thought it might be of interest of you.

Of course, there must be much more of these situations. Does anyone
know of other examples?

Peter S.