my mate phil says:
*"followed by caustic soda crystals
relatively dangerous bit:
eye and breathing masks and marigolds on, pour a mugful into the sinkhole and leave for 5 minutes. it'll fizz and start to heat up and produce toxic vapours.
realistically very dangerous bit, but effective:
return with a litre jug of just boiled water, double check your protective gear, and pour into the sink in one go. retreat.
once the reaction dies down, flush through with two sink bowl fulls of cold water
if caustic gets on surfaces or clothes it produces a soapy highly corrosive film, thoroughly wipe down any contaminated surfaces, still wearing your protective gear. put any contaminated clothes immediately into a cold wash or cold water bucket. if you get some on your skin, flush under cold water only for 5 minutes, there may still be chemical burning and scarring afterwards. if in the eye, flush under cold water and call an ambulance"*
he's a chemist, therefore a bit of a chancer though..
the quote function for tracts of text is shite