A perfect spot for a hunted otter to hide and rest, to give birth, to raise cubs.  The name for an otter’s hidey-hole is ‘holt’.

‘… she had swum away down the river, and hidden in the hollow of the water-lapped trunk.’

Source: Henry Williamson, Tarka the Otter: His joyful water-life and death in the two rivers, illus. C.F. Tunnicliffe (Harmondsworth: Puffin Books, 1976 (1927)), p. 16