The cobwebs stick to you, resisting your attempts to remove it from your hands, but otherwise appears to be just regular cobwebs.

Having reached the landing you can see the stairs continue for another 5' down before running into 10' of running water. There is a grate visible in the water to your right. It blocks passage that way. Every few seconds you can hear some mechanical grinding, a hiss and then a rush of water.

Marllow[spoiler]The lack of steam and heat leads you to believe that it is not boilers, but it might be hydraulics.[/spoiler]

Could be an elevator of some sort.  I imagine anyone who makes the journey regularly wouldn't want to climb every time

Cumulo steps down to the bottom of the ladder and takes a look around.

[spoiler]See anything?  Spot check and listen check
4(1d20) +4 = 8; 5(1d20) +4 = 9[/spoiler]

Marl will keep his lead as he sneaks ahead.

Stepping into the disgusting sewer water, you discover it is only two feet deep. From your new vantage point you are able to see the tunnel widens in the next section. The widened area has raised sides, each with water worn grooves below metal circles. Just then one of the circles opens, revealing itself to actually being a metal iris valve. When the valve opens, there is first some mechanical grinding, followed by a hiss and then a rush of water, it then closes. As you watch other irises open and close, but you are unable to discern any pattern.