 At the north end of the island is a half-submerged caldera, Brouton Bay, with an entrance only 2.5 meters deep, plunging to 240 meters in the center.In spite of its temperate latitude, the powerful Oyashio Current on the western flank of the Aleutian Low gives Simushir a chilly and very wet subarctic climate (Köppen "Dfc") that amazingly almost qualifies as a polar climate ("ET"), which in low-lying areas would be expected only at latitudes about 20 degrees or further north.Unlike typical subarctic or polar climates, however, the winters are only moderately severe and there is no permafrost since the mean annual temperature is around , whilst temperatures have never fallen below .