While working with 100 sponges to dry out an Ocean Monument I got them all wet and threw them in the Ovens installed on the top floor of the monument (kelp block furnace fuel). 

Upon logging back in a couple of hours later, the ovens are cold and there are no sponges to be found save the 1 I somehow managed not to pick up. Given that none of the other ocean monuments we've explored recently have had sponge rooms this is a truly unfortunate occurrence.