A Dyson tree is a hypothetical genetically-engineered plant, (perhaps resembling a tree) capable of growing in a comet, suggested by the physicist Freeman Dyson. He suggested that such plants could produce a breathable atmosphere within hollow spaces in the comet (or even within the plants themselves) utilising solar energy and cometary materials, thus providing self-sustaining habitats for humanity in the outer solar system.
A Dyson tree might consist of a few main trunk structures growing out from a comet nucleus, flowering into branches and leaves that intertwine, forming a spherical structure possibly dozens of kilometers across.
There’s nothing quite like these little scifi treats now and then. Also: megastructures.