Periodic Table and Boiling Points.

[JS_Princeton]

The same as liquid graphite. If I recall correctly, under high temperatures and pressures, carbon changes to graphite (which is a higher entropy structure) before any other physical change can occur.

[femtosecond]

No one knows for sure, because no one has really done it yet. But there are computer models.

Liquid diamond and carbon spaghetti : http://www.nature.com/nsu/990617/990617-10.html

The high-density form of liquid carbon is like a jumbled-up diamond – most of the atoms have four neighbours, but they are not arranged regularly as they are in diamond. The low-density form, meanwhile, is not like graphite at all, but contains mostly carbon atoms linked to just two others in chain-like structures: a kind of carbon spaghetti.