Stuart Thompson, from the RNLI, says one of the risks of tombstoning is the sheer unpredictability of the environment.
"The problem is that you know heights of tides change, conditions change" he says.
"The conditions of the seabed change as well so it's actually quite a dangerous practice".
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Seven Welsh sites are in the running to be Heathrow logistics hubs which would deliver a number of skilled and semi skilled jobs in Wales.
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Wales are looking for a new manager after Chris Coleman resigned in order to take the vacant job at Sunderland.