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The Shell Grotto lies largely under a residential garden at a depth of less than two metres. In the 1980s this garden was concreted over, resulting in much wetter conditions in the Grotto, which was placed on the Historic England Buildings at Risk Register in 1999.

We embarked on an ambitious conservation programme in 2007. Environmental monitoring showed relative humidity levels at more than 90%. Large areas of shells had delaminated from the chalk substrate; some areas had completely disintegrated and fallen away. To make matters worse, throughout the 20th century many ill-advised and frankly cack-handed repairs had been made to the mosaic. 

Survey work began and it became clear that one leaking pipe from a neighbouring property was responsible for a great deal of damage in the Rotunda. This was promptly re-lined, halting decades of deterioration, and we could start reinforcing the chalk around the Grotto, a carefully considered process of drilling holes and gravity grouting from above.

English Heritage favoured a natural turf roof on the Grotto so the next job was to set about breaking up the concrete roof, all 14 tonnes of it, delicately and by hand. With the green roof in place, the internal repairs could begin; the first time, after all this work, that the results would be visible to the public. 

Cleaning trials on the shells tested a number of possible solutions, ranging from water to white spirit and acetone. On heavily soiled areas, the only effective method was found to be a poultice soaked in a solution of ammonium carbonate, which was left overnight, with the dirt then lifted off using a cotton bud. It revealed that in most places the shells are white under the dirt; it was consequently felt that embarking on further cleaning would be unwise, considering the amount of moisture it would introduce.

Internal works continued with the pinning back of areas of shell decoration that had become detached from the chalk. This involved drilling a hole and screwing in stainless steel reinforcing bars, which were then resined into place. This process was repeated hundreds of times, and on each occasion the repair was carefully concealed and its location recorded. Meanwhile, work was carried out on the areas that had completely lost their shell decoration, replacing previous poor repairs and re-bedding some shells in damaged areas. For this we used shells that we had collected from the Grotto floor over many years.

Our conservators Rob Smith and Adrian Powell were nominated for the Heritage Angel Awards for their work here, reaching the finals. 

We continue to monitor environmental conditions, and we're now working on the Roundel Project.

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Visitors to the Grotto may notice bare slates scattered throughout the passages; these are areas where we believe the mosaic was worked above ground, and then the roundel placed into the Grotto’s design. The mortar has lasted less effectively on these roundels and there were 100 of them missing their shells back in 2012, when we embarked on the Roundel Project.


We commissioned conservator Rob Smith to begin recreating the missing mosaic pieces. The works are carried out in accordance with our Listed Building Consent, with shells carefully sourced and designs matched to early 20th-century photos of the Grotto. Washes of mineral paint are used to marry the new panels to the surrounding mosaic and roundels are a permanent addition to the Grotto’s fabric.



The Friends of the Shell Grotto are regularly funding roundels and visitors can also sponsor – or part sponsor – a roundel, either in your own name, as a business, or as a gift. And you can name a roundel in memory of a loved one. 


Sponsors’ names are displayed in the Museum Room and here on the Project page, with the roundel they have sponsored marked on a map. Sponsors also receive a verification certificate and an invitation to view their roundel once it is installed.

(approx. 10cm diameter)

(approx. 14cm diameter)

(approx. 18cm diameter)








- Friends of the Shell Grotto -

 - Minette Vickery -


- Carol Barber -

- Elijah’s Mermaid -

 - In Loving Memory of John
& Ruth Pearson -

- Marion & Cliff Edwards -

- Subterranea Britannica -

- In memory of the Hobson Family -

- In memory of Stephen Bateman -

- Katia Wengraf -


- Gordon Johnston -

- Eve, Rob and Baby Bump Lysander Holdsworth, Valentine’s Day 2017 -

- Masons Arms, Teddington -

- Shell Grotto Poster Patrons -

- Catherine & Rebecca Redmayne -

- Eric Love, remembered with love and deep affection -

 - Bessie Styles, from her loving grandchildren; Diane, David, Sandra, Susan & Sally-Anne -

- The Pettman Family -

- Friends of the Shell Grotto,
Vintage Film Night -

- The Coven of the Bright Isle -

- Amber Butchart & Rob Flowers -

- Nicky Paton, Royston Cave -

- Chris Pearson -

Thank you

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Formed in 2008, The Friends of the Shell Grotto is an independent trust committed to preserving and promoting this unique Grade I listed structure. Whilst run independently to the Grotto, the Friends work closely with the owners and play an essential role in the life and work of the Grotto. Everyone who becomes a Friend will be making an invaluable contribution to the Grotto’s future.


Membership starts at £15 per year and gives free unlimited entry to the Grotto, invitations to Friends’ events such as tours, talks and performances, regular updates on Grotto news and special deals at other attractions too.



(2 friends at the same address)

(2 adults and 2 children up to 16yrs)

(Life-long free entry and inclusion on
our Friends' Scroll at the Grotto)







- to promote, conserve and preserve the Shell Grotto as a unique historical monument

- to advance education by raising awareness of the historical value of the site

- to acquire, display, research and document historical artefacts and memorabilia

- to research and document the origins and subsequent uses of the Grotto

Membership Benefits

- unlimited Access

- talks & performances

- email newsletter

- volunteer with us

You can join us by clicking on the PayPal Donate button above and then entering the amount of your donation.

If joining as Joint or Family Friends, please include a note with the names you would like the membership cards to be in. You can also purchase membership as a gift simply by letting us know in the note.

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