Retired Navy Adm. James Stavridis thinks one of the most “lucrative” targets that terrorists could potentially attack is cruise ships with little in the way of defenses.

Stavridis, who retired from the Navy as the supreme allied commander in Europe, gave the cruise ship example as one of two scenarios that have him “very concerned” in an interview with The Cipher Brief.

Though Stavridis was mostly focused on the emerging sea power on display from Russia and China, he warned that terrorists could someday move weapons of mass destruction on container ships, or attack cruise liners, which typically carry upwards of 5,000 passengers.