Lake Nemrut fills the collapsed summit of a volcano above Tatvan, in Bitlis province, and it has a second lake beside it at bath temperature. The big one is cold enough that the authorities tell you not to swim in it: 176 metres deep, sitting at 2,247 metres. It was stocked with mirror carp only in 1986; before that there was almost nothing here to catch. A few hundred metres away, across black lava and grass, lies Ilıgöl. It is shallow and warm. Springs rise along a fault across the caldera floor and keep it six to eight degrees warmer than a lake at this altitude has any right to be. You wade in off the reeds onto a bottom that is soft, silty and startlingly warm underfoot, with the odd cooler current where a spring pushes through. From the rim road you get the whole caldera: deep blue in the main water, green in Ilıgöl, black lava between them.
The same heat does something stranger in winter. Steam leaking from the floor freezes as it escapes and builds hollow chimneys of ice above the snow. The mountain is dormant rather than finished. Armenian chronicles record the summit tearing open in 1441 with fire, smoke and a noise heard across the region, and two more eruptions were written down after that. Snow lies in the bowl into May, so the meadows here flower weeks behind the shores below. The colour comes in June and July, when the floor between the birch stands turns to open grass and the path runs through it to the reeds. For about ten days in the second half of October the birch and aspen go gold and bronze down to the water, often with the first snow already on the rim. What this volcano is really known for is downhill. Roughly 250,000 years ago it pushed more than sixty kilometres of lava across the Murat valley. What backed up behind that flow is Lake Van, six hundred metres below the rim you are standing on, and the largest lake in the country.