Tofua (56 square km) is a flat-topped volcanic island about 505 meters high with a steep and rocky shoreline all the way around. The 10 abandoned houses and three churches at Hokula near the north coast, and Manaka, a tiny settlement on the east coast, are used by villagers from Kotu Island, who come to harvest Tofua's potent kava. It takes about an hour to climb up to Tofua's rim from Hokula.
The large, steep-sided, four-km-wide caldera in the interior is occupied by a freshwater crater lake 30 meters above sea level and 250 meters deep. Tofua is still active: Steam and gases issue from a volcanic cone on the north side of the lake, and a hot pool is on the east side. Passing ships can see flames at night.
Until 1999, a seaplane based in Nuku'alofa ran charter flights to Tofua. The plane would make a photo pass over Kao and land on Tofua's crater lake. Passengers would then hike across the caldera for an hour to a smoking crater, and later there would be time for a swim in the lake. Unfortunately, there weren't enough customers and the operation has closed.
The only way to get to Tofua now is to charter a small boat from Lifuka or Nuku'alofa which can be very expensive. From Nuku'alofa, a fast speedboat will still take around four hours each way to cover the 145 km. Overnight trips usually involve sleeping ashore under a large tarpaulin tied between four trees. You must swim 10 meters to Tofua from the boat. If you do charter a boat to Tofua, ask if it will cost extra to cruise around Kao on your way back. You'll also pass the dormant volcanic islands of Hunga Tonga and Hunga Ha'apai on the way to/from Nuku'alofa.
This extinct 1,046-meter-high volcano, four km north of Tofua, is the tallest in Tonga; on a clear day the classic triangular cone is visible from Lifuka, 56 km east. There's no anchorage, but it's possible to land on the south side of the uninhabited island in good weather. The lower slopes are well wooded, becoming barren higher up. Kao can be climbed in a long day.