Olympic sized?
I figure Endurance Crater was actually a very large Martian swimming pool. As the water table went down the floatie with the tablets gradually sank down to the bottom depositing more and more stabilzed (very) chlorine into the the plaster. Knew those fractured shelfs looked like something familiar. If only we had a shovel on Oppy. We could dig down a bit and find the rebar.
(sorry, it was just too good a straightline)