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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Organisers say there will be increased security at the Urdd National Eisteddfod, which began in Bridgend today.
Any remaining heavy or thundery showers will clear eastwards this evening.
Over 70 firefighters are currently tackling the fire, which is now confirmed to be at a scrap yard.