Banks are required to file Suspicious Activity Reports for any suspicious (very broadly defined) deposits or withdrawals even if the amount is much less than $10k. The feds can also temporarily designate certain geographic regions as 'high risk' which means banks in those regions must comply with much more stringent reporting requirements.

It is also illegal to move money in such a way as to evade the reporting requirements ('structuring', what they nailed Dennis Hastert for).