Step 1: Understanding the Concept:
The question asks to identify the author of the seminal work "Mare Liberum," a foundational text in the development of international law, particularly the Law of the Sea.
Step 2: Detailed Explanation:
Hugo Grotius (1583-1645), a Dutch jurist and philosopher, is widely regarded as one of the fathers of modern international law. In 1609, he published \textit{Mare Liberum} (The Freedom of the Seas) anonymously. In this short treatise, he formulated the new principle that the sea was international territory and all nations were free to use it for seafaring trade. This was a direct challenge to the claims of various naval powers to exclusive sovereignty over vast areas of the sea (\textit{mare clausum} or closed sea). The principles laid down by Grotius in this book became the bedrock of the modern doctrine of the freedom of the high seas.
- Arvid Pardo is famous for proposing the "common heritage of mankind" principle for the seabed in the 20th century.
- Austin and Starke are well-known jurists, but not the authors of this 17th-century work.
Step 3: Final Answer:
The author of \textit{Mare Liberum} is Hugo Grotius.