The "midst" of something in the Bible represents the thing that is most central and most important to the spiritual state being described, the motivation that drives everything else. In general this will be something we love or feel, because at the core of things we are what we love; our loves define us.

(References: Apocalypse Revealed 44, 90, 268; Arcana Coelestia 2252, 2940, 10153, 10365, 10557, 10635, 10641)

8872. 'And which are in the waters under the earth' means which exist on the level of the bodily senses. This is clear from the meaning of 'a likeness of the things which are in the waters under the earth', as those which come below the ones that exist in natural light. The fact that they exist on the level of the bodily senses is evident from the consecutive degrees of the light that shines in a person's understanding. In the first degree with him there are the things which exist in spiritual light; these are meant by 'the things which are in the heavens above'. In the second degree there are the things which exist in natural light; these are meant by 'the things which are on the earth beneath'. And in the third degree there are the things which exist on the level of the bodily senses; these are meant by 'the things which are in the waters under the earth'. What the level of the bodily senses is and the nature of it, see 5084, 5089, 5094, 5125, 5128, 5580, 5767, 6183, 6201, 6310-6316, 6318, 6598, 6612, 6614, 6622, 6624, 6844, 6845, 6948, 6949, 7442, 7693. The things which come to be known most directly from the experience of the outward senses, and the delights accompanying such knowledge, are connected with the level of the bodily senses. With those who are good that knowledge and associated delights are good because they are used in good deeds; but with those who are bad they are bad because they are used in bad deeds. Misleading by means of such knowledge and its delights, as pretenders, hypocrites, and deceivers are accustomed to do, is 'making a likeness of such things as are below the waters under the earth.'