Where is the entry to the underground place of power in Velen?
South of Midcopse and north of Byways there is a place of power. It's near an crumbled tower on which a Noonwraith roams. It seems to be underground yet I cannot find its entrance anywhere around.
Go east to the sea and follow the shore to the south:
You will then come across a cave entry. It's the same cave that is part of a sidequest with Keira Metz.
Inside you find the Place of Power and also a hidden treasure after a bit of climbing.
There's also a video guide on YouTube: