A shell is a company without assets.  Being a "shell risk" is typical of exploration-phase mining companies.  However, many simply go bankrupt, leaving a shell of a company (as an asset) to be picked up by someone else.  I know that's the way it works in Canada.  I'm not sure about the US. 

Try this:

https://www.investopedia.com/terms/s/shellcorporation.asp