It is still uncertain whether the cosmic censorship conjecture is true or not. To get a new insight into this issue, we propose the concept of the border of spacetime as a generalization of the spacetime singularity and discuss its visibility. The visible border, corresponding to the naked singularity, is not only relevant to mathematical completeness of general relativity but also a window into new physics in strongly curved spacetimes, which is in principle observable.PACS numbers: 04.20. Dw, 04.20.Cv In general relativity, the singularity theorems tell us that spacetime singularities exist in generic gravitational collapse spacetime (see e.g., Ref.[1]). For singularities formed in gravitational collapse, Penrose [2,3] proposed the so-called cosmic censorship conjecture, which has two versions. For spacetimes which contain physically reasonable matter fields and develop from generic nonsingular initial data, the weak one claims that there is no singularity which is visible from infinity, while the strong one claims that there is no singularity which is visible to any observer. A singularity censored by the strong version is called a naked singularity, while a singularity censored by the weak version is called a globally naked singularity. There is no general proof for the conjecture at present. Recent development on critical behavior ([4, 5], see also Ref. [6]) and self-similar attractor ([7], see also Ref. [8]) has shown that there are naked-singular solutions which result from nonsingular initial data and contain physically reasonable matter fields. The critical solution has one unstable mode while the self-similar attractor has no unstable mode against spherical perturbation, although it is still uncertain whether these examples are stable against all other possible perturbations. If all examples of naked singularities were shown to be unstable, would it mean that they are all rubbish?We have already known that general relativity will have the limitation of its applicable scale in a high-energy side. A simple and natural discussion on quantum effects of gravity yields the Planck energy E Pl ∼ 10 19 GeV as a cut-off scale Λ. Some theories with large extra dimensions may have much lower cut-off scale, which could be TeV scale [9, 10]. The energy scale of the curved spacetime can be measured by the curvature through Einstein's field equations. Then if the above expectation is true, general relativity is not applicable to the spacetime region whose curvature strength exceeds Λ 4 /E 2 Pl . This consideration naturally leads to the notion of the border of spacetime as follows. Let (M, g) be a spacetime manifold M with a metric g. We call a spacetime region A ⊂ M a border if and only if the following inequality is satisfied:where the curvature strength F is given, for instance, byWe denote the union of all borders in M by U B . We call a border A a visible border if and only ifis not empty, whereWe can also naturally define a globally visible border. To make the definition precise, we assume that the spacetime ...