Much of the tourist industry is concentrated along the Adriatic Sea coast. Opatija was the first holiday resort. It first became popular in the middle of the 19th century. By the 1890s, it had become one of the most significant European health resorts. Later many resorts sprang up along the coast and islands, offering services catering to mass tourism and various niche markets. The most significant are nautical tourism, as there are marinas with more than 16 thousand berths, cultural tourism relying on the appeal of medieval coastal cities and cultural events taking place during the summer. Inland areas offer agrotourism, mountain resorts, and spas. Zagreb is also a significant tourist destination, rivalling major coastal cities and resorts.