They seems to be regular travel about as far south as Nova Scotia. Some explorers have traveled farther south (to the US West Coast and even to South America. possibly) where they've witnessed glowing stubs of cities and experienced radiation poisoning along the "Black Coasts".

The implication seems to be that New York and the western coastal cities were almost totally obliterated in a nuclear holocaust.

The Black Coasts would appear to be an extreme form of Badlands. Since any close approach to them is likely to be fatal nothing can be said of them with certainty but that they are entirely barren, and in some regions are known to glow dimly on a dark night.

and

‘Uncle Axel, are there any cities there?’
‘Cities?’ he repeated. ‘Well, here and there you’ll find a town, of a kind. As big as Kentak, maybe, but built differently.’
‘No,’ I told him. ‘I mean big places.’ I described the city in my dream, but without telling him it was a dream.
He looked at me oddly. ‘No, I never heard of any place like that,’ he told me. ‘Farther on, perhaps. Farther than you went?’ I suggested.
He shook his head. ‘You can’t go farther on. The sea gets full of weed. Masses of weed with stems like cables. A ship can’t make her way through it, and it’s trouble enough to get clear of it once you get in it at all.’

and

‘This is a dreadful country indeed. We have seen Badlands before, but none of us has ever imagined anything quite so terrible as this. There are stretches, miles across, where it looks as if all the ground has been fused into black glass; there is nothing else, nothing but the glass like a frozen ocean of ink… then belts of Badlands… then another wilderness of black glass. It goes on and on… What did they do here? What can they have done to create such a frightful place?… No wonder none of us ever came this way before. It’s like going over the rim of the world, into the outskirts of hell… it must be utterly beyond hope, barred to any kind of life for ever and ever… But why? – why? – why?… There was the power of gods in the hands of children, we know: but were they mad children, all of them quite mad?… The mountains are cinders and the plains are black glass – still, after centuries!… It is so dreary… dreary… a monstrous madness… It is frightening to think that a whole race could go insane…. If we did not know that you are on the other side of it we should have turned back and fled—’

Past the Badlands, as SQB has noted in his answer are areas where there are people with black skin and what appear to be primates. It's not clear whether this is South America or Southern America or the Caribbean but it's described as being found after "hundreds of miles" of blackened coastline.

For hundreds of miles the coast goes on being Badlands with stretches of the dead, black lands; so far, in fact, that the first ships down there gave up and turned about because they thought they would never reach any place where they could water and provision. They came back saying that they thought it must go on like that to the ends of the earth.

They seems to be regular travel about as far south as Nova Scotia. Some explorers have traveled farther south (to the US West Coast and even to South America. possibly) where they've witnessed glowing stubs of cities and experienced radiation poisoning along the "Black Coasts".

The implication seems to be that New York and the western coastal cities were almost totally obliterated in a nuclear holocaust.

The Black Coasts would appear to be an extreme form of Badlands. Since any close approach to them is likely to be fatal nothing can be said of them with certainty but that they are entirely barren, and in some regions are known to glow dimly on a dark night.

and

‘Uncle Axel, are there any cities there?’
‘Cities?’ he repeated. ‘Well, here and there you’ll find a town, of a kind. As big as Kentak, maybe, but built differently.’
‘No,’ I told him. ‘I mean big places.’ I described the city in my dream, but without telling him it was a dream.
He looked at me oddly. ‘No, I never heard of any place like that,’ he told me. ‘Farther on, perhaps. Farther than you went?’ I suggested.
He shook his head. ‘You can’t go farther on. The sea gets full of weed. Masses of weed with stems like cables. A ship can’t make her way through it, and it’s trouble enough to get clear of it once you get in it at all.’

and

‘This is a dreadful country indeed. We have seen Badlands before, but none of us has ever imagined anything quite so terrible as this. There are stretches, miles across, where it looks as if all the ground has been fused into black glass; there is nothing else, nothing but the glass like a frozen ocean of ink… then belts of Badlands… then another wilderness of black glass. It goes on and on… What did they do here? What can they have done to create such a frightful place?… No wonder none of us ever came this way before. It’s like going over the rim of the world, into the outskirts of hell… it must be utterly beyond hope, barred to any kind of life for ever and ever… But why? – why? – why?… There was the power of gods in the hands of children, we know: but were they mad children, all of them quite mad?… The mountains are cinders and the plains are black glass – still, after centuries!… It is so dreary… dreary… a monstrous madness… It is frightening to think that a whole race could go insane…. If we did not know that you are on the other side of it we should have turned back and fled—’

Past the Badlands, as SQB has noted in his answer are areas where there are people with black skin and what appear to be primates. It's not clear whether this is South America or Southern America or the Caribbean but it's described as being found after "hundreds of miles" of blackened coastline.

For hundreds of miles the coast goes on being Badlands with stretches of the dead, black lands; so far, in fact, that the first ships down there gave up and turned about because they thought they would never reach any place where they could water and provision. They came back saying that they thought it must go on like that to the ends of the earth.

They appear to have traveled about as far south as Nova Scotia. Some explorers have traveled farther south (to the US West Coast) where they've witnessed glowing stubs of cities and experienced radiation poisoning along the "Black Coasts".

The implication seems to be that New York and the western coastal cities were almost totally obliterated in a nuclear holocaust.

The Black Coasts would appear to be an extreme form of Badlands. Since any close approach to them is likely to be fatal nothing can be said of them with certainty but that they are entirely barren, and in some regions are known to glow dimly on a dark night.

and

‘Uncle Axel, are there any cities there?’
‘Cities?’ he repeated. ‘Well, here and there you’ll find a town, of a kind. As big as Kentak, maybe, but built differently.’
‘No,’ I told him. ‘I mean big places.’ I described the city in my dream, but without telling him it was a dream.
He looked at me oddly. ‘No, I never heard of any place like that,’ he told me. ‘Farther on, perhaps. Farther than you went?’ I suggested.
He shook his head. ‘You can’t go farther on. The sea gets full of weed. Masses of weed with stems like cables. A ship can’t make her way through it, and it’s trouble enough to get clear of it once you get in it at all.’

and

‘This is a dreadful country indeed. We have seen Badlands before, but none of us has ever imagined anything quite so terrible as this. There are stretches, miles across, where it looks as if all the ground has been fused into black glass; there is nothing else, nothing but the glass like a frozen ocean of ink… then belts of Badlands… then another wilderness of black glass. It goes on and on… What did they do here? What can they have done to create such a frightful place?… No wonder none of us ever came this way before. It’s like going over the rim of the world, into the outskirts of hell… it must be utterly beyond hope, barred to any kind of life for ever and ever… But why? – why? – why?… There was the power of gods in the hands of children, we know: but were they mad children, all of them quite mad?… The mountains are cinders and the plains are black glass – still, after centuries!… It is so dreary… dreary… a monstrous madness… It is frightening to think that a whole race could go insane…. If we did not know that you are on the other side of it we should have turned back and fled—’

They seems to be regular travel about as far south as Nova Scotia. Some explorers have traveled farther south (to the US West Coast and even to South America. possibly) where they've witnessed glowing stubs of cities and experienced radiation poisoning along the "Black Coasts".

The implication seems to be that New York and the western coastal cities were almost totally obliterated in a nuclear holocaust.

The Black Coasts would appear to be an extreme form of Badlands. Since any close approach to them is likely to be fatal nothing can be said of them with certainty but that they are entirely barren, and in some regions are known to glow dimly on a dark night.

and

‘Uncle Axel, are there any cities there?’
‘Cities?’ he repeated. ‘Well, here and there you’ll find a town, of a kind. As big as Kentak, maybe, but built differently.’
‘No,’ I told him. ‘I mean big places.’ I described the city in my dream, but without telling him it was a dream.
He looked at me oddly. ‘No, I never heard of any place like that,’ he told me. ‘Farther on, perhaps. Farther than you went?’ I suggested.
He shook his head. ‘You can’t go farther on. The sea gets full of weed. Masses of weed with stems like cables. A ship can’t make her way through it, and it’s trouble enough to get clear of it once you get in it at all.’

and

‘This is a dreadful country indeed. We have seen Badlands before, but none of us has ever imagined anything quite so terrible as this. There are stretches, miles across, where it looks as if all the ground has been fused into black glass; there is nothing else, nothing but the glass like a frozen ocean of ink… then belts of Badlands… then another wilderness of black glass. It goes on and on… What did they do here? What can they have done to create such a frightful place?… No wonder none of us ever came this way before. It’s like going over the rim of the world, into the outskirts of hell… it must be utterly beyond hope, barred to any kind of life for ever and ever… But why? – why? – why?… There was the power of gods in the hands of children, we know: but were they mad children, all of them quite mad?… The mountains are cinders and the plains are black glass – still, after centuries!… It is so dreary… dreary… a monstrous madness… It is frightening to think that a whole race could go insane…. If we did not know that you are on the other side of it we should have turned back and fled—’

Past the Badlands, as SQB has noted in his answer are areas where there are people with black skin and what appear to be primates. It's not clear whether this is South America or Southern America or the Caribbean but it's described as being found after "hundreds of miles" of blackened coastline.

For hundreds of miles the coast goes on being Badlands with stretches of the dead, black lands; so far, in fact, that the first ships down there gave up and turned about because they thought they would never reach any place where they could water and provision. They came back saying that they thought it must go on like that to the ends of the earth.