diff --git a/explorer/testdata/addr/fail-method-let.carbon b/explorer/testdata/addr/fail_method_let.carbon
similarity index 93%
rename from explorer/testdata/addr/fail-method-let.carbon
rename to explorer/testdata/addr/fail_method_let.carbon
index 5d6866d9f651..5e3091a4ac06 100644
--- a/explorer/testdata/addr/fail-method-let.carbon
+++ b/explorer/testdata/addr/fail_method_let.carbon
@@ -28,7 +28,7 @@ class Point {
 
 fn Main() -> i32 {
   let p: Point = Point.Origin();
-  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/addr/fail-method-let.carbon:[[@LINE+1]]: method GetSetX requires its receiver to be an lvalue
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/addr/fail_method_let.carbon:[[@LINE+1]]: method GetSetX requires its receiver to be an lvalue
   var x: auto = p.GetSetX(42);
   if (p.x == 42) {
     return x;
diff --git a/explorer/testdata/addr/fail-method-me-misspelled.carbon b/explorer/testdata/addr/fail_method_me_misspelled.carbon
similarity index 91%
rename from explorer/testdata/addr/fail-method-me-misspelled.carbon
rename to explorer/testdata/addr/fail_method_me_misspelled.carbon
index eb6f0a563e5b..83ab8aed9dfc 100644
--- a/explorer/testdata/addr/fail-method-me-misspelled.carbon
+++ b/explorer/testdata/addr/fail_method_me_misspelled.carbon
@@ -11,7 +11,7 @@
 package ExplorerTest api;
 
 class C {
-  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/addr/fail-method-me-misspelled.carbon:[[@LINE+1]]: illegal binding pattern in implicit parameter list
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/addr/fail_method_me_misspelled.carbon:[[@LINE+1]]: illegal binding pattern in implicit parameter list
   fn F[addr mew: Self*]() {}
 }
 
diff --git a/explorer/testdata/addr/fail-method-me-type.carbon b/explorer/testdata/addr/fail_method_me_type.carbon
similarity index 94%
rename from explorer/testdata/addr/fail-method-me-type.carbon
rename to explorer/testdata/addr/fail_method_me_type.carbon
index 3babe10f6a83..976ab85adfd4 100644
--- a/explorer/testdata/addr/fail-method-me-type.carbon
+++ b/explorer/testdata/addr/fail_method_me_type.carbon
@@ -32,7 +32,7 @@ class Point {
 
 fn Main() -> i32 {
   var p: Point = Point.Origin();
-  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/addr/fail-method-me-type.carbon:[[@LINE+1]]: type error in method access, receiver type: 'class Point' is not implicitly convertible to 'class Shape'
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/addr/fail_method_me_type.carbon:[[@LINE+1]]: type error in method access, receiver type: 'class Point' is not implicitly convertible to 'class Shape'
   var x: auto = p.GetSetX(42);
   if (p.x == 42) {
     return x;
diff --git a/explorer/testdata/addr/nested-method.carbon b/explorer/testdata/addr/nested_method.carbon
similarity index 100%
rename from explorer/testdata/addr/nested-method.carbon
rename to explorer/testdata/addr/nested_method.carbon
diff --git a/explorer/testdata/mixin/fail-circular-mixing.carbon b/explorer/testdata/mixin/fail_circular_mixing.carbon
similarity index 92%
rename from explorer/testdata/mixin/fail-circular-mixing.carbon
rename to explorer/testdata/mixin/fail_circular_mixing.carbon
index 7b1144eb305f..13bf3d19506c 100644
--- a/explorer/testdata/mixin/fail-circular-mixing.carbon
+++ b/explorer/testdata/mixin/fail_circular_mixing.carbon
@@ -13,7 +13,7 @@ __mixin M1 {
   fn F1[me: Self](x: Self) -> Self{
      return x;
   }
-  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail-circular-mixing.carbon:[[@LINE+1]]: 'M3' has not been declared yet
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_circular_mixing.carbon:[[@LINE+1]]: 'M3' has not been declared yet
   __mix M3;
 }
 
diff --git a/explorer/testdata/mixin/fail-field-member-name-clash.carbon b/explorer/testdata/mixin/fail_field_member_name_clash.carbon
similarity index 81%
rename from explorer/testdata/mixin/fail-field-member-name-clash.carbon
rename to explorer/testdata/mixin/fail_field_member_name_clash.carbon
index 1c1464fef91c..f0e5f6acfbee 100644
--- a/explorer/testdata/mixin/fail-field-member-name-clash.carbon
+++ b/explorer/testdata/mixin/fail_field_member_name_clash.carbon
@@ -19,7 +19,7 @@ __mixin M {
 class C {
   let F: i32 = 0;
   __mix M;
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail-field-member-name-clash.carbon:[[@LINE+1]]: Member named F (declared at {{.*}}/explorer/testdata/mixin/fail-field-member-name-clash.carbon:16) cannot be mixed into C because it clashes with an existing member with the same name (declared at {{.*}}/explorer/testdata/mixin/fail-field-member-name-clash.carbon:20)
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_field_member_name_clash.carbon:[[@LINE+1]]: Member named F (declared at {{.*}}/explorer/testdata/mixin/fail_field_member_name_clash.carbon:16) cannot be mixed into C because it clashes with an existing member with the same name (declared at {{.*}}/explorer/testdata/mixin/fail_field_member_name_clash.carbon:20)
 }
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/mixin/fail-method-member-name-clash.carbon b/explorer/testdata/mixin/fail_method_member_name_clash.carbon
similarity index 76%
rename from explorer/testdata/mixin/fail-method-member-name-clash.carbon
rename to explorer/testdata/mixin/fail_method_member_name_clash.carbon
index a43575bb6cca..2b18c62df677 100644
--- a/explorer/testdata/mixin/fail-method-member-name-clash.carbon
+++ b/explorer/testdata/mixin/fail_method_member_name_clash.carbon
@@ -21,7 +21,7 @@ class C {
      return x;
   }
   __mix M;
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail-method-member-name-clash.carbon:[[@LINE+1]]: Member named F (declared at {{.*}}/explorer/testdata/mixin/fail-method-member-name-clash.carbon:16) cannot be mixed into C because it clashes with an existing member with the same name (declared at {{.*}}/explorer/testdata/mixin/fail-method-member-name-clash.carbon:22)
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_method_member_name_clash.carbon:[[@LINE+1]]: Member named F (declared at {{.*}}/explorer/testdata/mixin/fail_method_member_name_clash.carbon:16) cannot be mixed into C because it clashes with an existing member with the same name (declared at {{.*}}/explorer/testdata/mixin/fail_method_member_name_clash.carbon:22)
 }
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/mixin/fail-mix-as-type-expr.carbon b/explorer/testdata/mixin/fail_mix_as_type_expr.carbon
similarity index 91%
rename from explorer/testdata/mixin/fail-mix-as-type-expr.carbon
rename to explorer/testdata/mixin/fail_mix_as_type_expr.carbon
index d4f369ee9cb5..b1fa73cdeed9 100644
--- a/explorer/testdata/mixin/fail-mix-as-type-expr.carbon
+++ b/explorer/testdata/mixin/fail_mix_as_type_expr.carbon
@@ -13,7 +13,7 @@ package ExplorerTest api;
 __mixin Operations {}
 
 fn Main() -> i32 {
-   // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail-mix-as-type-expr.carbon:[[@LINE+1]]: Expected a type, but got mixin Operations
+   // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_mix_as_type_expr.carbon:[[@LINE+1]]: Expected a type, but got mixin Operations
    var a: Operations;
    return 0;
 }
diff --git a/explorer/testdata/mixin/fail-mix-diamond-clash.carbon b/explorer/testdata/mixin/fail_mix_diamond_clash.carbon
similarity index 86%
rename from explorer/testdata/mixin/fail-mix-diamond-clash.carbon
rename to explorer/testdata/mixin/fail_mix_diamond_clash.carbon
index 77eadcb269c1..e7b7bf5318bf 100644
--- a/explorer/testdata/mixin/fail-mix-diamond-clash.carbon
+++ b/explorer/testdata/mixin/fail_mix_diamond_clash.carbon
@@ -30,7 +30,7 @@ __mixin M3 {
 class C {
   __mix M2;
   __mix M3;
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail-mix-diamond-clash.carbon:[[@LINE+1]]: Member named F1 (declared at {{.*}}/explorer/testdata/mixin/fail-mix-diamond-clash.carbon:15) is being mixed multiple times into C
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_mix_diamond_clash.carbon:[[@LINE+1]]: Member named F1 (declared at {{.*}}/explorer/testdata/mixin/fail_mix_diamond_clash.carbon:15) is being mixed multiple times into C
 }
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/mixin/fail-mix-in-global.carbon b/explorer/testdata/mixin/fail_mix_in_global.carbon
similarity index 94%
rename from explorer/testdata/mixin/fail-mix-in-global.carbon
rename to explorer/testdata/mixin/fail_mix_in_global.carbon
index 21ab9ac4bd32..b472eae7d2ea 100644
--- a/explorer/testdata/mixin/fail-mix-in-global.carbon
+++ b/explorer/testdata/mixin/fail_mix_in_global.carbon
@@ -16,7 +16,7 @@ __mixin Operations {
   }
 }
 
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail-mix-in-global.carbon:[[@LINE+1]]: syntax error, unexpected MIX, expecting END_OF_FILE
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_mix_in_global.carbon:[[@LINE+1]]: syntax error, unexpected MIX, expecting END_OF_FILE
 __mix Operations;
 
 class Point {
diff --git a/explorer/testdata/mixin/fail-mix-in-impl.carbon b/explorer/testdata/mixin/fail_mix_in_impl.carbon
similarity index 92%
rename from explorer/testdata/mixin/fail-mix-in-impl.carbon
rename to explorer/testdata/mixin/fail_mix_in_impl.carbon
index cfa561db6f1b..7b0c44be46dd 100644
--- a/explorer/testdata/mixin/fail-mix-in-impl.carbon
+++ b/explorer/testdata/mixin/fail_mix_in_impl.carbon
@@ -21,7 +21,7 @@ interface A {
 }
 
 external impl i32 as A {
-  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail-mix-in-impl.carbon:[[@LINE+1]]: syntax error, unexpected MIX, expecting ALIAS or FN or RIGHT_CURLY_BRACE
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_mix_in_impl.carbon:[[@LINE+1]]: syntax error, unexpected MIX, expecting ALIAS or FN or RIGHT_CURLY_BRACE
   __mix Operations;
   fn F() {}
 }
diff --git a/explorer/testdata/mixin/fail-mix-members-clash.carbon b/explorer/testdata/mixin/fail_mix_members_clash.carbon
similarity index 83%
rename from explorer/testdata/mixin/fail-mix-members-clash.carbon
rename to explorer/testdata/mixin/fail_mix_members_clash.carbon
index f73ea3717757..183e345d7e20 100644
--- a/explorer/testdata/mixin/fail-mix-members-clash.carbon
+++ b/explorer/testdata/mixin/fail_mix_members_clash.carbon
@@ -23,7 +23,7 @@ __mixin M2 {
 class C {
   __mix M1;
   __mix M2;
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail-mix-members-clash.carbon:[[@LINE+1]]: Member named F1 (declared at {{.*}}/explorer/testdata/mixin/fail-mix-members-clash.carbon:20) cannot be mixed into C because it clashes with an existing member with the same name (declared at {{.*}}/explorer/testdata/mixin/fail-mix-members-clash.carbon:15)
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_mix_members_clash.carbon:[[@LINE+1]]: Member named F1 (declared at {{.*}}/explorer/testdata/mixin/fail_mix_members_clash.carbon:20) cannot be mixed into C because it clashes with an existing member with the same name (declared at {{.*}}/explorer/testdata/mixin/fail_mix_members_clash.carbon:15)
 }
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/mixin/fail-recursive-mixing.carbon b/explorer/testdata/mixin/fail_recursive_mixing.carbon
similarity index 85%
rename from explorer/testdata/mixin/fail-recursive-mixing.carbon
rename to explorer/testdata/mixin/fail_recursive_mixing.carbon
index 6fe5080214d5..70267364ecb1 100644
--- a/explorer/testdata/mixin/fail-recursive-mixing.carbon
+++ b/explorer/testdata/mixin/fail_recursive_mixing.carbon
@@ -14,7 +14,7 @@ __mixin M1 {
      return x;
   }
   __mix M1;
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail-recursive-mixing.carbon:[[@LINE+1]]: Member named F1 (declared at {{.*}}/explorer/testdata/mixin/fail-recursive-mixing.carbon:15) is being mixed multiple times into M1
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_recursive_mixing.carbon:[[@LINE+1]]: Member named F1 (declared at {{.*}}/explorer/testdata/mixin/fail_recursive_mixing.carbon:15) is being mixed multiple times into M1
 }
 
 class C {
diff --git a/explorer/testdata/mixin/fail-self-substitution.carbon b/explorer/testdata/mixin/fail_self_substitution.carbon
similarity index 94%
rename from explorer/testdata/mixin/fail-self-substitution.carbon
rename to explorer/testdata/mixin/fail_self_substitution.carbon
index 614974344b90..cd117b0a8434 100644
--- a/explorer/testdata/mixin/fail-self-substitution.carbon
+++ b/explorer/testdata/mixin/fail_self_substitution.carbon
@@ -38,7 +38,7 @@ class Complex {
 fn Main() -> i32 {
   var p: Point = Point.Origin();
   var c: Complex = {.r = 42, .i = 1};
-  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail-self-substitution.carbon:[[@LINE+1]]: type error in call: 'class Complex' is not implicitly convertible to 'class Point'
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/mixin/fail_self_substitution.carbon:[[@LINE+1]]: type error in call: 'class Complex' is not implicitly convertible to 'class Point'
   var p1: Point = p.F(c);
 
   return p1.x - 42;
diff --git a/explorer/testdata/mixin/self-substitution.carbon b/explorer/testdata/mixin/self_substitution.carbon
similarity index 100%
rename from explorer/testdata/mixin/self-substitution.carbon
rename to explorer/testdata/mixin/self_substitution.carbon
diff --git a/explorer/testdata/mixin/simple-mix-in-mixin.carbon b/explorer/testdata/mixin/simple_mix_in_mixin.carbon
similarity index 100%
rename from explorer/testdata/mixin/simple-mix-in-mixin.carbon
rename to explorer/testdata/mixin/simple_mix_in_mixin.carbon
diff --git a/explorer/testdata/mixin/simple-reuse.carbon b/explorer/testdata/mixin/simple_reuse.carbon
similarity index 100%
rename from explorer/testdata/mixin/simple-reuse.carbon
rename to explorer/testdata/mixin/simple_reuse.carbon
diff --git a/explorer/testdata/mixin/use-mixin-method-in-class-method.carbon b/explorer/testdata/mixin/use_mixin_method_in_class_method.carbon
similarity index 100%
rename from explorer/testdata/mixin/use-mixin-method-in-class-method.carbon
rename to explorer/testdata/mixin/use_mixin_method_in_class_method.carbon