Portal

Almost certainly to an incomprehensible alien world: most are in my experience. It is curious how, somehow, the atmosphere is always breathable: a recent paper speculates that portals between worlds with radically-incompatible biospheres are somehow corroded by one or both atmospheres and fail, usually resulting in the abrupt destruction of both biospheres as nasty things leak through. 'Abrupt' is actually an exaggeration: unless one or both biospheres contain things extremely toxic in small doses to the other it takes several decades for enough to leak through an opening the size of a door.

The Agency assumes that biological contamination is unlikely to be a problem in this sense – alien viruses are just too, well, alien, countless movies notwithstanding. There remains the question of worlds whose atmospheres consist of nerve agents, say.