Flash foods in Portstewart left people shaken as surging water created holes in the road.
Drivers were forced to turn around to avoid huge pools and a "fountain" of water, with some onlookers saying the road surface was falling apart before their eyes this afternoon (July 4).
Julia Anderson posted a video of the flash flooding on Twitter.
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She said that the water was causing the road surface to "swell up" and "crumble".
She told this website the experience of seeing the strength of the water had left her shaken up.
She said: "First we saw a hole that had burst and created a mini fountain in the middle of the road."
That was at 5.29pm, when Julia took pictures of a bubbling fountain of water which had burst out of the road surface (see above picture).
But just two minutes later - at 5.31pm - she said the road started "rising up".
She added: "Then the road started rising up so we had to make a U-Turn in the middle of the road.
"Once we got out we were a bit shaken but okay."
Neil Rodgers also shared footage of flooding on the promenade.
His video appears to show overflowing drains struggling to cope with the deluge of water delivered by the thunderstorm.
Murky water can be seen spilling out and rising slowly around a nearby car and motorhome.
Neil simply said: "Ok Portstewart promenade didn't handle the thunderstorm well..."
In more bad news, the Met Office is predicting heavy rain will return to the area on Monday afternoon (July 5).
The week will start off cloudy with light rain, before stronger downpours arrive for a couple of hours from 3pm, the forecaster says.
It is also saying that light rain will be likely on Tuesday (July 6) and Wednesday (July 7) in Portstewart.
A PSNI Spokesperson said: "Portstewart Promenade is closed due to structural damage caused by adverse weather.
"Please avoid the area as there is no access to vehicles or pedestrians.
"Please be advised that the road is expected to remain closed for some time."
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