That hole is almost the entire heartwood missing, rotted out after some damage some years ago. The heartwood is there for support, and without it the tree is more likely to be unstable in adverse weather conditions. However, if the top of the tree, the canopy, were to be reduced significantly, and you could be sure that if the tree fell it would do no damage to buildings/persons walking by etc. then the risk is yours to take. Personally I would be happy that it was a sycamore and not a particularly special tree, and accept the fact that it would be better removed.

Tree holes can be caused by an injury to the tree, such as breakage of a limb and a hole is created opening the bark and exposing the sapwood. Fungi and bacteria will then attacks the sapwood which form a cavity in the hole of the tree. There are wet and dry tree holes , the wet type of cavity can then fill with water and become a type of phytotelma (plant water body). Looking at how yours has formed, there doesn't seem to be a cavity where the water can pool, so I ewould say it's a dry type.

Looking at the picture I have a theory but I might be wrong, which is that it's actually two trees that have grown very close together and the bark at the back has fused together giving the appearance of a single tree. I know that two trees or branches of adjacent trees can rub together and graft into each other but I don't know if this could create what you have?

I would remove it to be honest , problems for the future

Gg

Its only a young one anyway - do you really want a mature sycamore in your garden, making shade and producing hundreds of seedlings every year? If you want a big tree of this sort why not replace it with one of the nicer maples? I'd look on this one as a source of firewood.
(How can you tell we are surrounded by them here?)

I agree Steragram, get rid of it. I removed mine some years back as it was a nightmare. I had some problems from the local council after the event because apparently I shouldn't have chopped it down because is was over 25 years old and thus protected or something like that. I just told them that it was diseased and causing a problem and they just said ok, that's fine ;o)

get rid of it - sycamores have no place in a domestic garden. Way too big, drip honeydew on everything and send out seeds like nobody's business. Great in woodland or open park land though