An old friend of mine, determined to make his fortune, decided it would be a splendid idea to make an amphibious beach buggy, them being all the rage at the time. We bought an old VW, took the body off and had a lot of fun zooming round his field with just the seat & steering column held in place by an axle stand. We built a mould and nearly gassed ourselves laying up a fibreglass tub in his garage. We got as far as fixing this to the rolling chassis, but we'd given no thought to how we might seal the axles etc. Of course, it was totally impractical.
The tub languished in a corner of my friend's yard, until someone had the bright idea of sinking it in the ground to make an inspection pit. We duly dug the hole, and mixed up some weak concrete to act as a surround. Being outside, this more or less immediately filled with rain and, being very thick fibreglass, proved difficult to make drainage holes in. As it happened, my friend had a firearms license, so we had fun making holes with his Browning 9mm hi-power.
I've no idea what became of this pit. It soon became too unpleasant to use, filling up with mud & rotting vegetation. Probably still there.