How do your neighbors deal with the problème? Where on your property is it located, the back or near the house?
I would consider to fence in the eyesore with a buxus hedge for example. Bonne chance et courage:-)

Could you take a picture of where these are in relation to your house -- also what style is the home? Would putting a decorative fence/screen around them work for you? Or some bushes?

Merci. We have no neighbours. Most fosse septique systems are flush with the ground but when ours was being dug, rock was discovered under the sub soil so it could not sunk into the ground. It is so awful that the inspector from the water company who came to review it's technical working said of the 5000 systems he has inspected, this is the most horrible. Unfortunately, it is only 2.5 metres from the back door.

The house is a 250 year old traditional cottage.

Can you incorporate it into a flower bed? We have a similar system in Ohio and I p,anted around it (while leaving it accessible) and then popped some planters on the tops of the tanks. It's not perfect but, when life gives you lemons.......

They are a lovely shade of green, they are beautifully cylindrical, they are a necessity. Embrace them as part of your beautiful space. Cut a garden bed around them and put stones around them so that they become a sculptural addition to your land.
You could block your view of them with whatever local shrubs and trees you can find that you can purchase to create a symmetrical planting on the side that is visible from the house.
You could also paint the tops in a whimsical faux pebble design or just make light of the situation.
J'aimerais etre en France pour vous aiderais! Votre terre est formidable!
Bonne chance.

Any sort of landscape structures or boulders as mentioned would help disguise the eye sores.

Are you planning on building a deck of any sort on the back of the house? If so, these covers would probably end up near the outside edge. Perhaps you could incorporate them into the landscape design by having a sort of Japanese style ( removable) walking bridge that takes you from the deck to another part of the garden. You could make this "bridge" look like it is crossing a dry riverbed. Just look up images of Japanese style gardens and I think you will get the idea. I don't think it will be a big obstacle for you with some thoughtful planning.

Do you have the option of bringing in soil to be flush with the caps? You'd need a fair bit to be able to feather the grade out so it doesn't look like someone buried an elephant but it's the simplest fix. That's going to help you blend the caps in with the rest of the landscape better than trying to wall off around them with a hedge (which will only call attention to them)

I have a gîte in France, but have never had that problem. The only suggestion is have decking above it with a hatch above the necessary places.

Never fear, the ground around it will settle and grass over by next summer with a minor amount of work (levelling and sprinkling in grass seed and watering), but I would incorporate a rockery with shrubs in the immediate area around the three caps (which is pretty much what a number of other helpful people have suggested). At my parents' place in France they ended up with one really obvious hatch right in the middle of their beautiful lawn. My mum was devastated, but the track to it - that originally looked like a giant mole had been hard at work - disappeared by the end of the spring and she artfully place an old wheelbarrow with a pot of geraniums on the crazy and unsightly hatch. Et voila!

painted up to look like toadstools?

Hey there, why don't you buy hollow rock garden ornaments to put over them? This way it will look natural in your garden - here's an example

I'd make a raised wooden bed around it, with beautiful lavender. Perhaps fixed on three sides and the fourth side could be pulled in and out if they really must get into them. That side I'd make as lightweight as possible, even use fake plants so you don't break your back pulling it out. Don't despair, there's always a way. Enjoy France :-)

With that view, I'd just put a large bench over them. Job done.

Have you checked whether the three projecting cylidrical pieces cut be cut and the inspection covers resited at a lower level? Some drainage products are designed in this way. If this is possible it would make it easier to deal with.

Put a table and chairs over it?

I do appreciate the suggestions ,thank you so much everyone for taking the trouble to share your ideas. I think we will go with the suggestion made by Revolutionary Gardens and create a gentle slope. I was particularly taken by the image of the buried elephant, I would probably have created what looked like a cemetery for several dinosaurs without your wise words. We have so much land around these projections that I can begin the slope at least 100 feet away on every side. For those who have suggested a table and or chairs over the offending projections, the idea makes perfect sense except for the small point that these hatches lead directly into our main sewage system and on a warm day, with the wind in a particular direction, the aroma emanating from them does rather put you off your dinner. Hope the eyesore will be a bit less overwhelming by next spring so will post some pics of how it looks then.

I have a fosse right behind my house in France. It is under a concreted over terrace. There are two steps up, it's only about 2m from the house. I would suggest you do as some others have suggested and build a terrace flush with the tops. I have an old one with only one access. I have covered the whole thing with gravel and no one knows it's there. We had it inspected when we moved in, 2003, and never opened it since! It's obviouly a soak away type. I am amazed there is a smell. Get your installers back, there shouldn't be any smell. I have quite similar weather to you here in Charente Maritime.
I know this is rather after the fact, you might have solved your problem by now. Enjoy your life in France.

We had one in the garden in the UK and we made it part of a new bed. We planted tall grasses, Russian Sage and Verbena. The height was very good and the swaying of the grasses was beautiful in the wind. Looks beautiful almost all year round. Grasses are so varied you can pick your own height and colours to suit your taste palette. Bees loved it as did butterflies. Best wishes Anna

I have a similar dilemma with an lpg hatch which is in the middle of the front garden but I need to access it once/ twice a year . Looking for ideas please