This year’s winner of the Yale Series of Younger Poets competition is Davis McCombs’s Ultima Thule, which was acclaimed as “a book of exploration, of searching regard.... a grave, attentive holding of a light” by the contest judge, the distinguished poet W. S. Merwin. The poems are set above and below the Cave Country of south central Kentucky, where McCombs lives and which is home to thousands of caves. The book is framed by two sonnet sequences, the first about a slave guide and explorer at Mammoth Cave in the mid-1800s and the second about McCombs’s experiences as a guide and park ranger there in the 1990s. Other poems deal with Mammoth Cave’s four- thousand-year human history and the thrills of crawling into tight, rarely visited passageways to see what lies beyond. Often the poems search for oblique angles into personal experience, and the caves and the landscape they create form a personal geology.