As for physical security, some are only protected by simple manhole covers before reaching the actual technical building, here's an example in Marseille, France where SeaMeWe-4 (and others) lands: 43.261938, 5.37276 [0] but the landing points aren't exactly secret [1]. Here are some photos [2][3] and a video [4] of Dunant's arrival in France.
[0] https://www.geoportail.gouv.fr/carte?c=5.372494831681247,43....
[1] https://www.sigcables.com/index.php/cableliste/fiche_cable/5...
[2] https://twitter.com/jlvuillemin/status/1238414261774401537
[3] https://twitter.com/jlvuillemin/status/1238433769935319042
[4] https://twitter.com/jlvuillemin/status/1238479381145751553