# Tunnels trail random question

**URL:** <https://spokes.nz/t/tunnels-trail-random-question/1645>\
**Category:** General\
**Tags:** dunedin-tunnels-trail\
**Created:** [24 May 2026 03:45 UTC](https://spokes.nz/t/tunnels-trail-random-question/1645 "2026-05-24T03:45:53Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![jessica](https://spokes.nz/letter_avatar_proxy/v4/letter/j/73ab20/32.png) [@jessica](https://spokes.nz/u/jessica)\
**Post date:** [24 May 2026 03:45 UTC](https://spokes.nz/t/tunnels-trail-random-question/1645/1 "2026-05-24T03:45:53Z")

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![IMG_7254](https://spokes.nz/uploads/default/original/2X/0/024f4210c90abe43ba94c32de8576910e5e81a1e.jpeg)

checked out the tunnels trail. Just a random question; what’s on the walls? My friend that it was mould, but I assume it’s all checked out by the council. Couldn’t find any info about it on the tunnels trails website.

Great walk / ride though! 👏

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**Author:** ![GerardH](https://spokes.nz/user_avatar/spokes.nz/gerardh/32/213_2.png) [@GerardH](https://spokes.nz/u/GerardH)\
**Post date:** [25 May 2026 03:32 UTC](https://spokes.nz/t/tunnels-trail-random-question/1645/2 "2026-05-25T03:32:25Z")

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Calcite deposits built up over 150 years! Looks slimy but while wet it’s actually hard.

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**Author:** ![Robyn](https://spokes.nz/user_avatar/spokes.nz/robyn/32/1026_2.png) [@Robyn](https://spokes.nz/u/Robyn)\
**Post date:** [25 May 2026 04:47 UTC](https://spokes.nz/t/tunnels-trail-random-question/1645/3 "2026-05-25T04:47:02Z")

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Wonder where the calcium carbonate comes from?

Usually when it’s dark like that it’s got lots of iron in it, but maybe it’s the dark bricks behind it. I’m used to seeing it on pale limestone walls

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**Author:** ![jessica](https://spokes.nz/letter_avatar_proxy/v4/letter/j/73ab20/32.png) [@jessica](https://spokes.nz/u/jessica)\
**Post date:** [25 May 2026 09:37 UTC](https://spokes.nz/t/tunnels-trail-random-question/1645/4 "2026-05-25T09:37:40Z")

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Thanks Gerard. Good to know. 👍

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**Author:** ![NathanK](https://spokes.nz/user_avatar/spokes.nz/nathank/32/2002_2.png) [@NathanK](https://spokes.nz/u/NathanK)\
**Post date:** [25 May 2026 20:37 UTC](https://spokes.nz/t/tunnels-trail-random-question/1645/5 "2026-05-25T20:37:29Z")

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> [@Robyn](#):
>
> Wonder where the calcium carbonate comes from?

I wondered this myself.

Lime (i.e. ground up limestone) is a key ingredient in cement, so it might be simply from slowly leaching out of the the mortar between and behind the bricks.