diff --git a/backend/explore/cache_test.go b/backend/explore/cache_test.go
index 8feb4d2..a1bc0a4 100644
--- a/backend/explore/cache_test.go
+++ b/backend/explore/cache_test.go
@@ -46,23 +46,42 @@ func TestCacheMiss(t *testing.T) {
 	}
 }
 
+// TestCacheTTLExpiry checks both halves of the TTL contract, and uses two
+// entries to do it.
+//
+// **No assertion here may depend on an upper bound of elapsed wall-clock
+// time**, which is what the single-entry version of this test did: it set
+// a 1s TTL and immediately asserted a *hit*, so on a loaded runner — one
+// goroutine descheduled for over a second while the rest of the suite
+// runs — the entry was correctly gone and the test failed with "expected
+// cache hit immediately after set".  It did exactly that in CI while
+// passing five times out of five locally.
+//
+// Sleeping *past* a TTL is always safe, so the expiry half keeps a short
+// one; the presence half gets a TTL nothing can outrun.
 func TestCacheTTLExpiry(t *testing.T) {
 	c := newTestCache(t)
 
 	data := []byte(`{"ephemeral":true}`)
-	c.Set("ttl-test-key", data, 1*time.Second, "", "")
+	c.Set("ttl-live-key", data, time.Hour, "", "")
+	c.Set("ttl-expiring-key", data, 1*time.Second, "", "")
 
-	// Verify it's there immediately.
-	if _, ok := c.Get("ttl-test-key"); !ok {
-		t.Fatal("expected cache hit immediately after set")
+	if _, ok := c.Get("ttl-live-key"); !ok {
+		t.Fatal("expected a cache hit on an entry with an hour to live")
 	}
 
-	// Wait for expiry.
+	// Wait for the short one to expire.
 	time.Sleep(2 * time.Second)
 
-	if _, ok := c.Get("ttl-test-key"); ok {
+	if _, ok := c.Get("ttl-expiring-key"); ok {
 		t.Error("expected cache miss after TTL expiry, got hit")
 	}
+
+	// And the long-lived entry is still there, which is what says the
+	// sweep above expired an entry rather than the cache.
+	if _, ok := c.Get("ttl-live-key"); !ok {
+		t.Error("the hour-long entry expired too")
+	}
 }
 
 func TestCacheMBID(t *testing.T) {