Wearside's flagship leisure centre pool is open again after it had to be cleaned in the aftermath of an "incident."
The facility at Sunderland Aquatic Centre, in Stadium Park, Monkwearmouth, closed yesterday afternoon and remains shut today.
Its management has said it is now open to visitors once again following tests.
Its bosses said the closure is down to "unforeseen circumstances," and said an incident happened yesterday, but have not given any further details of what happened.
Steve Dougal, Everyone Active’s general manager at the centre, said: “I can confirm that an incident did occur at the Sunderland Aquatic Centre on Monday, April 10, which led to the main pool being temporarily closed for cleaning.
"This procedure has been carried out to ensure we maintain our standard of water quality for all users.
"We apologise for any inconvenience this may cause to our visitors and appreciate their patience during this time.
"Please visit our Facebook and Twitter pages for updates on the pool’s opening or contact the centre directly on 03330 050151.”
Last night's swimming lessons and public session were cancelled, but the aqua fit class, learn to dive lesson and activity camps, which are held in the smaller pool in the complex, went ahead.
The centre has said payments taken by Direct Debit for lessons will be adjusted to reflect the cancelled lessons.