An opportunity to investigate in advance of new construction led to the discovery of five Mesolithic hazel fish traps some 6.3m below mean sea level in the River Liffey. Closely paralleled on the continent of Europe they imply a well organised community that knew how to catch fish using the tide, to make wattle-work and baskets and who undertook coppicing on an eight year cycle in about 6100-5700 cal BC. The likelihood of more Mesolithic remains under European towns that have remained attractive to fishers and settlers has considerable implications for Cultural Resource Management. Do we always know how to find and access such delicate and important traces?