Investigation work has started ahead of a scheme to build a £500m deep sea container terminal at Avonmouth Docks in Bristol.
Preparations involve an off-shore rig drilling through the seabed to check the strength of the rock.
It will give engineers information to help improve the final plans for the terminal to be partially built on land reclaimed from the River Severn.
The terminal will be able to take some of the world's biggest container ships.
The Bristol Port Company said Costain and BAM Nuttall would help "update existing records" during the detailed site investigation.
The firm has estimated more than 1,200 jobs could be created as a result of the development.