Frostbreaking
From Frostpunk 2 Wiki and Guides
This is the mechanic used to clear hexagonal tiles so that districts can be placed.
Usually 8 Tiles must be chosen to frostbreak at a time, indicated by the octagonal divisions near the mouse cursor. If 8 Tiles cannot be chosen because the aren't any more connecting tiles (in case of "Islands"), Frostbreaking will still cost full price and time. It costs about 40 heatstamps and reserves 300 workforce for 2 weeks.
Resource deposits under the ice will be highlighted, but pressing ALT key will make these deposits even easier to see. These are useful because they are required for certain districts.