Mandatory reporting of foreign assets on Form TD F 90-22.1
If the total of all your foreign bank accounts is $10,000 or higher, then you must complete Treasury Department Form TD F 90-22.1 and send it in early enough to be received by June 30 each year.
If you don't do it because you didn't realize you had to, you could be fined up to $10,000. If it's determined that you deliberately concealed the information, the fine could reach $100,000 or 50% of the money in the accounts.
The form is available for download here: TD F 90-22.1 and should not be mailed with your tax return. Instead, send it to:
US Department of the Treasury
P.O. Box 32621
Detroit, MI 48232-0621