On earth, such rising lakes of warm water are caused by volcanoes under the ice. Something similar may happen on Europa:
Here we report an analysis of archival data from Europa, guided by processes observed within Earth's subglacial volcanoes and ice shelves. The data suggest that chaos terrains form above liquid water lenses perched within the ice shell as shallow as 3 kilometres. Our results suggest that ice–water interactions and freeze-out give rise to the diverse morphologies and topography of chaos terrains. The sunken topography of Thera Macula indicates that Europa is actively resurfacing over a lens comparable in volume to the Great Lakes in North America.This is getting a lot of attention because life would be more likely in that sealed ocean if there were a way for it to make contact with space. The solar system is full of simple organic molecules, especially around Jupiter, and they must rain down on Europa's surface. If they could get into the comparatively warm ocean, they would promote life. The new theory proposes a mechanism whereby the surface of Europa would slowly be recycled, and anything falling on the surface would eventually be incorporated into the ocean below.