A fleet of almost 300 ships without international identifiers traveled to the waters just off North Korea’s west-coast city of Haeju for months last year to dredge up tons of ocean sand before returning to China, a new report released Friday revealed.

Sand is a resource prohibited for export from North Korea according to UN sanctions, and the dredging operation, detailed in up-close satellite images and ship activity heat maps by the U.S.-based organization C4ADS, could constitute a major sanctions breach.

According to C4ADS, 279 unique Automatic Identification System (AIS) numbers representing various kinds of ships were found to