It’s an astonishing peak, and sacred to Hindus and Sasaks who make pilgrimages to the summit and lake to leave offerings for the gods and spirits. To the Balinese, Rinjani is one of three sacred mountains, along with Bali’s Agung and Java’s Bromo. Sasaks ascend throughout the year around the full moon.
The mountain has climatic significance. Its peak attracts a steady stream of swirling rain clouds, while its ash emissions bring fertility to the island’s rice fields and tobacco crops, feeding a tapestry of paddies, fields, and cashew and mango orchards.
The official website of Gunung Rinjani National Park has good maps, info and a useful section on reported scams by dodgy hiking operators. Note that trekking independently up Gunung Rinjani is not allowed.
According to lonelyplanet.com