Apparently part of London's subway network causes problems for kohanim because it is underneath a museum that includes human remains:

Due to the Science Museum and the Underground entrance sharing the same roof, at present, Kohanim are unable to use South Kensington station and any underground line that passes through it.

There is, therefore, currently a proposal to build an extra structure outside the entrance to the station: you can see before-and-after pictures here. I've found an explanation which says:

The proposed structure would be set to left and right of the entrance to the subway, with a partial horizontal return above set just under the heavy existing cornice and forming a small roof structure. It is understood that the intention is symbolically to sever the entrance to the station from the building above, which houses human remains. This would allow access to the station to Kohanim. To be effective in this regard it has to extend beyond the outermost line of the cornice to the roof above, and so projects out from the entrance frontage beyond the elevation of the porch structure which it adjoins, although only on the right hand side.

My question is: how does this 'work' halachically? Looking at the pictures, I can't understand how the partial archway (covering only half of the entrance, ie the right-hand side) would suffice to prevent tumat met from flowing into the subway network. Can anyone figure it out?

  • "tumat met... flowing into the subway network". Does it flow? Why did you choose that word? EDIT: I just saw your other question, fair enough
    – Rabbi Kaii
    Jun 14, 2023 at 12:00
  • @RabbiKaii Seems like a perfectly apt word. Eg Mishnah Ohalot 14:2 talks about tumah travelling through gaps and openings of certain sizes. What word would you use?
    – Zarka
    Jun 14, 2023 at 12:05
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    @DoubleAA Probably not necessary to comment if all you have to say is that you don't know?
    – Zarka
    Jun 18, 2023 at 12:59
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    @Shmuel Absolutely. That’s why I’m wondering how the archway helps.
    – Zarka
    Jun 18, 2023 at 17:49
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    I'm just trying to help so you don't sit around much waiting. You may obviously do as you please. Good luck!
    – Double AA
    Jun 18, 2023 at 20:12

1 Answer 1


It seems that tumah is being transferred via the grey ledge. Perhaps that ledge extends to entrance to the museum both to the right and to the left.

If that is the case tumah would spread from the entrance of the museum to the area under the ledge (if there is a tefach width). From there it would spread to the entrance to the subway.

The proposed black structure would stop the tumah from spreading to the entrance of the subway.

It takes a bit more guesswork to figure out why the black structure needs to extend over the entrance. Perhaps there is something above the entrance, some kind of structure jutting out from the building above. The black structure could be extending beyond the reach of the upper structure and blocking the tuma

So this would be one scenario which would explain how it would suffice.

  • That’s really helpful - it hadn’t occurred to me that the vertical parts of the archway might be the key. When you say ‘some kind of structure jutting out from the building above’, presumably you mean that it only juts out above where the horizontal part of the black archway is?
    – Zarka
    Jun 18, 2023 at 18:55
  • @Zarka correct. I still feel something is missing...
    – Mark
    Jun 19, 2023 at 8:40

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