The poem is only a few lines long, yet it pulls in two directions at once: down into the earth and out into the strangeness of a dream. In the first line, "we fit ourselves to the dirt" suggests miners shaping their bodies and lives to the narrow, dark space they work in. The speaker says "we," so the feeling is shared, almost like a quiet acceptance of the job. Then "walk down the mine" is flat and matter-of-fact, with no drama in it.
The last two lines shift the scene. The speaker has "left my keys to the field / in a dream." Keys usually mean access or control, and here they are lost in an open field, far from the mine. One reading is that the speaker has given up some freedom or way back to the surface. The mood is hushed, a little uneasy, and dreamlike. Nothing is explained, so the reader is left with a sense of descent and of something important being left behind.