800 Meg /bin/core file??

[Ben Cox]

Hi,
  I was happily copying some directory structures from one machine to
another when my FTP client came across a file called 'core' in my
tomcat's bin directory.  This file was (and is) 821,313,536 bytes. 
Anyone have the slightest idea what this file is and how it would grow
so large?
  Thanks in advance,

    Ben