diff --git a/Test-problems.md b/Test-problems.md
index 16cf674..dd1a5f9 100644
--- a/Test-problems.md
+++ b/Test-problems.md
@@ -381,4 +381,16 @@ $$s + t = 1 + \tfrac{1}{2} st$$
 
 #### Note
 
-[Geometry Expressions](https://geometryexpressions.com) can solve this problem handily!
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+[Geometry Expressions](https://geometryexpressions.com) can solve this problem handily!
+
+## Other ambient geometries
+
+### Crystal structures
+
+#### Source
+
+Peter Fenyes
+
+#### Statement
+
+Assemble triply periodic sphere packings like the ones sometimes used to illustrate [crystal structures](https://chem.libretexts.org/Courses/East_Tennessee_State_University/CHEM_3110%3A_Descriptive_Inorganic_Chemistry/05%3A_Structure_and_Energetics_of_Solids/5.01%3A_Crystal_Structures_and_Unit_Cells).
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