Missing bolt on 200tdi crankcase

[Plutei]

While rummaging around today I discovered that there is a bolt missing from the crankcase. Seems to be 9/16, 35mm deep, not sure if it's a blind hole, anyone know what should be there?
Photo taken from the front underneath

Edit: photo is upside down :rocket:

[bee utey]

Wading plug. You put it in before crossing deep water and take it out for normal road service. It's where small crankshaft oil leaks become apparent before wrecking the timing belt.