I heard about this reading a book, and it seems this is common in Catholic parts of the US, but I’ve never seen any myself (mainly because Texas is very Baptist). It seems it consists of a bathtub stuck into the ground vertically halfway up, and then a Mary statue is placed in it.
I’ve not seen many bathtubs in Taiwan but a lot of gongyu had those short bathtubs that are often green (my grandparent’s house had this bathtub that consisted of brickwork made into a bathtub shape, lined with tiles). It seems bathtubs went out of style in Taiwan for some odd reason…
But I thought it would be funny if someone erected a bathtub Mary out of those discarded half length bathtubs usually ripped out of houses in Taiwan…
Well, during my 12 years of Catholic schooling in Texas never heard about this. Sounds like a poor man’s method to protect the statue from the elements.