In everyday play
The Hollows are not caves under the ground. They are the other side of the world, and a door dug down from the surface opens onto them.
A door down
Brackenholm keeps one of those doors, and elsewhere one can be dug. Open it by hand and the way leads down. The same door, climbed back to and opened, leads up into daylight.
Death below
A Vessel that dies in the Hollows is gone, as it would be anywhere, with everything it carried left where it fell, far from daylight.
Go prepared
It is dark below and it stays dark. Know the way back before you go further, and think about who you take with you. This guide does not say what lives there or what can be found.
Read The Hollows for the wider picture.