Mount Semeru, Java's highest peak and an active volcano within Bromo Tengger Semeru National Park, offers a challenging yet rewarding trekking experience typically requiring 2-3 days, accessed via Ranu Pani. All visits strictly require official permits and a licensed guide; solo ascents are prohibited for safety. The optimal time to visit is during the dry season (May to October), as the park is usually closed for climbing during the rainy season (November to April) due to hazardous conditions. Costs vary but generally include national park fees (around IDR 220,000-320,000 for foreigners), guide and porter services, and logistics, easily summing up to several hundred USD. Trekkers need excellent physical fitness, proper cold-weather gear, and must adhere to park regulations, often limiting ascent to a safe distance from the active crater due to gas and ash emissions.