The tanker sank on Tuesday with thousands of tonnes of oil still inside, and people are worried that it could still leak out.
The tanker is deep under the sea where it's really cold, and experts hope the oil will have turned solid and won't leak out.
But they're not sure - which is why the sub is going down to see what's going on.
Only one slick has hit the Spanish coast, but another four are being driven towards Spain by high winds.
The oil that has already hit the coast has caused 100 beaches to be closed and has put loads of fisherman out of work.
The Spanish government has stopped all fishing until things get better.