In a statement, the company – which employs around 300 people, with the majority at its five mines across Scotland – said it could not meet its debts after lenders demanded full repayment.
ATH's debt was recently bought by a fund controlled by venture capital firm Better Capital.
It was subsequently announced ATH was working on a sale or restructuring of the business with accountancy firm KPMG.
It was confirmed last night that administrators from KPMG have now been appointed.
ATH subsidiary Aardvark is not affected and continues to trade.