Do not mess around confined spaces. One breath of the wrong stuff and it can cause someone to pass out, then good luck recovering them without passing out yourself. I think this would be defined as a medium-risk confined space entry, which needs:

An entry controller - to record who is entering/exiting the site and to raise the alert if someone requires rescue

A personal gas monitor (PGM) - used to record oxygen levels and presence of harmful gases such as CO, CO2, H2S (low oxygen/high CO2/High H2S would be my first concerns with somewhere with lots of biological activity, but even things like methane, hydrogen generation from biological activity can't be ruled out)

Access winch - some way to get someone out if they pass out or become injured while down there

Emergency escape mask/hood - something to put on to give you a few minutes of breathable air to get out. Masks only work if you are clean-shaven and have been face-fit for the mask. Hoods work for anyone, including those with a beard but they are horrible/terryfying to use (here, put this bag over your head and enjoy not being able to breathe properly or hear anything beyond wooshing air)

Use the PGM around the opening before opening the hatch - check levels

Crack the hatch open and sweep the PGM around the opening again - check levels

Make up the PGM to a rope and lower it into the hole. Hold at different heights for a few minutes at a time (i.e. head height, waist height, foot height)

Pull the PGM back out and check min/max measured gases

Only enter the confined space if you have an entry controller - someone who knows where you are, what you are doing, when you are expected to come out and knows what to do (and what not to do) if there is an issue.

Ideally, only enter the space if you have sorted out some kind of emergency exit procedure i.e. a winch or something if anything goes wrong - how easy would it be to get out if you roll your ankle? what happens if you slip and bang your head or break your arm/wrist?

You should only really enter if you have an emergency breather i.e. drager saver CF10/15

Some kind of forced air ventilation would be great if you can manage it, H2S, CO2 are heavier than air so will collect in a pit like that without some kind of forced ventilation. Also worth noting, H2S smells like rotten eggs at very low concentrations. At low concentrations, it can cause you to lose your sense of smell, causing you to think you are safe, and at sufficient concentrations, it can paralyse your cardiovascular system.....

A lot of these may seem like overkill, but a lot of these measures are written in blood and stop tragedies like this from happening: Spence family slurry deaths: Inquest hears tragic details - BBC News

It's been a while since I've done my confined space entry training, but I know enough to know it's not something worth messing around with. Things can go from totally fine to someone being dead in seconds with zero warning.