Mount Semeru (3,676m) is Java's highest mountain and one of Indonesia's most active volcanoes — erupting on a near-daily basis with ash plumes visible from Bromo, 20 kilometres to the north. The standard trekking route begins at Ranu Pani lake (2,100m), passes through alpine meadows and forest, and reaches the summit in a two to three-day return. Trekking permits are required and issued at the Ranu Pani ranger post.
The trek is not technical but involves long uphill sections on volcanic sand that slides underfoot. The summit dawn view — over Java's highlands to the coast on both sides — is spectacular when cloud allows. The eruptions from the summit crater are visible on the approach route in the afternoon. Guides are strongly recommended for the final summit section. Ranu Pani is reached by jeep from Malang (2.5 hours) via Tumpang.