subglacial

adj

1. (not-comparable)Formed or occurring beneath a glacier or other body of ice.

   “These streams continue to flow in winter as well as in summer, although in less quantity; […] It is also this melting beneath the ice (subglacial, suppose we call it) which loosens the ice from the ground, and occasions, or rather permits, the perpetual downward movement, with which "The glacier's cold and restless mass / Moves onward day by day."”

2. (excessive, informal, not-comparable)Moving at a rate allegedly slower than a glacier; extremely slow.

   “Securing political approval for even modest changes to railway organisation and structure has been of sub-glacial speed and complexity since 2004, when Government abolished the Strategic Rail Authority and put itself in charge.”