Island disguise collapses when weight accumulates

Lyngbakr

When enough weight has accumulated on its back, or when the Lyngbakr decides the time is right, it suddenly dives deep into the ocean, throwing off the sailors and dragging their ships down into the depths.

Imap Umassoursa

It is described as resembling a flat island or low reef while it rests at or near the surface — a shape convincing enough that boats could mistake it for a real landform.