This page of Assassin's Creed Valhalla game guide answers the question of how to unlock access to wells found in the game world. These offer an entrance to underground areas.
While traveling around the game world you can find wells covered with boards. After clearing the boards, you'll be able to enter them.
Accessing wells is easy - use any projectile to destroy the boards covering them. A single hit should be sufficient to unlock access.
Unlocking wells enables you to get access to the treasures and passages hidden underground. These are marked when using Odin's Vision. After locating some underground treasure, start looking for a well or an entry point to access it. .