The guided climb up these twin volcanoes is reasonably demanding and the view is often obscured by low cloud and volcanic smoke and ash. But on a clear day the reward for your climb will be peering into an active crater and seeing the red-hot magma boiling below like a satanic pot of tomato soup. Both volcanoes are closely monitored, climbing is occasionally suspended on high-activity days and evacuation plans are always ready.
To make the climb, you must be reasonably fit and comfortable walking on steep terrain, and you need a good guide (compulsory). Skin protection and plenty of drinking water are essential. There’s a dry, slippery crust around both volcanoes so your boots need to be strong enough to kick toe holes. Ankle support is also necessary as there’s some boulder hopping. Between the mountains, the walk over the razor-backed ridge gets very narrow and snakes nastily upwards, while vents all around spurt acrid smoke. Mother Nature does her best to be daunting.
The best idea is to trek up one way and down another (different guides will meet you on the caldera). There are three routes, two accessed from Craig Cove airport and one from Ulei airport:
North From Ranon or Ranvetlam, this is the only option for going up and back (Mt Marum only) in one day. From Ranon to North Camp it's two hours through jungle, then another three hours to the top. Return, or continue down (with camping) on the south or east routes.
Southwest From Port Vatu or Lalinda it's five to six hours slogging through jungle to the ash plain and the West Camp (overnight), then a steep 1½-hour trek to Mt Benbow or 2½ hours to Mt Marum, from where you can continue down north or east.
East From Endu, 45 minutes north of Ulei airport, it’s a testing 6½-hour trek to the ash plain, across Wisal River to East Camp. From there it's just 45 minutes to the top of Mt Marum. Return or continue north to Ranon or south to Port Vatu.