Petrified Wood, wood that has been fossilized by being turned into stone. Petrifaction is a natural process that may form fossils from the solid remains of any plant or animal.
What would you need to explore an ocean on Europa, one of Jupiter's moons? Among other things, you'd need a submersible vehicle to explore the ocean and relate findings back to Earth. Stone Aerospace is developing just such a vehicle: the DEPTHX.