Got surprised with a wither health bar while exploring. Looks like it is below the ocean floor, didn't see the wither itself only its healthbar and could hear it breaking blocks.
Don't have the means to just kill it, yet so please be careful in that area. And staff might want to investigate

Coords are about 560, -100
*edit* main server, new world

Investigate a wither boss? are you mad?

Weeeeeeell i suppose we are, so yeah! i'll try to find out whats going on. Could be someones underground wither cage.

update, we killed it, but would love to know who was so careless in the overworld

Yeah - it took a LOT of effort to get rid of, and many, many deaths! (at least on my part)

Many thanks to everyone that helped out

For those not online, I found it in an underwater cave system (had to dig down on the ocean floor to find it). Looks like the cave system had already been explored as there were torches inside already (along with a Wither, gradually mining it's way through). In our attempts to kill it, it somehow escaped and headed towards land (slowly, as we were taking it in turns to be "target practice" for the Wither, whilst we were in boats / swimming!).