The next morning we ended up anchored at Del EnCalitro near the only port on Forementera.  It was a lovely spot, good views and only four yachts. 
The boat to the west of us left shortly after our arrival and we both commented on the huge ball of weed on his anchor as he up-anchored.
Ah! Memories of Falmouth! 

Posidonea - of course, silly!

25 knot winds were forecast to return the next day so we decided to stay with the boat just in case. We are still learning to trust our anchor – maybe we never will trust it!

Well, it was a good decision. As the winds built (to more than 25 knots by the way) we found we were slipping on the anchorage having been rigid solid for over 20 hours.

We made 3 attempts to re-anchor at various places in the bay.
Every time, the anchor slipped under engine power. The anchor kept getting caught in this #@#$ weed!
It took John several minutes each time to go and untangle huge masses of weed from the anchor each time. The last failure ended up with over ½ cubic meter of weed on the anchor!

We don’t need this!  I rang the port.  

The marina had just one spot left for a boat of our size. After very careful consideration of all the facts and issues pertaining to the proposal and taking into account all the views of all parties with a vested interest and following a careful risk analysis and ..  - I said we would be there in five minutes! 

This is where we learnt all about Posidonia.
It’s not a weed but an incredible organism that helps the environment and is a hundred thousand years old. Posidonia.  I’m sure that this is gonna be useful information for Trivial Pursuit one day.
In fact this ‘meadow’ around the coast here is all based upon ONE plant and is actually 100,000 years old!
But it still LOOKS like a weed!

Posidonia forms a carpet many many square miles wide over the sand, it is extremely dense and strong - hence the reason we could not get a good anchor or find any sand.  Actually Posidonia is protected in many areas of the Islands coastal anchorages now because it protects the beaches and keeps the water crystal clear. Again, not too much mention of this in the pilot books but we are on the look out for it now and avoiding it at all cost.

The marina at Formentera is a working ferry port.
Although it was a bit noisy it was well run and the staff were very attentive.
Luckily, we had a great spot on a pontoon away from the mass of other boats.