Pulau Una Una should really be called Pulau Colo, as that's the name of the volcano that encompasses most of the island and spectacularly exploded in 1983.
Today it's a popular stop for diving trips and less so for snorkelling, but it's also possible to arrange to climb the peak. To do so you'll need to charter a boat -- we were quoted 800,000 to a million rupiah for a boat from Kadidiri Paradise. You'll then need to arrange a local guide on Una Una, which we were told would cost an additional 300,000 rupiah.
While the peak doesn't look all that massive (compared to a more typical volcano anyway) we were told the climb is challenging and at the summit you're able to see into the caldera and its sulfur clouds. One climber we talked to said that it involves climbing up through dry waterfalls, which can be treacherously slippery after rain and which climbers should really be using equipment to climb. In 2012 a German traveller slipped and fell while climbing and broke her spine -- she needed to be carried out by hand. When we asked the climber if he'd climb it again he laughed and said, "No way!"
Should you plan to climb it, getting a guide on Una Una is strongly advisable -- bear in mind also that over Ramadan you may find it difficult to find a guide who really knows the way (there are two routes to the summit) and who isn't fasting.
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